George is nearly 13. He wanders into the kitchen, naked. He climbs on to the Aga and sits there twiddling a piece of cardboard. I send him to get dressed; his skin is red and mottled from the heat. He returns with all his clothes on the wrong way round.

I fill a lunchbox for 11-year-old Sam. Plain crisps, gluten-free biscuits, marzipan, an apple that I know he won't eat, but I suppose I live in hope. George doesn't have a lunchbox, because George maintains the fiction that he doesn't eat anything, and a lunchbox is too blatant a reminder that this cannot be the case. I smuggle his food supplies - mainly Twiglets and chocolate - into his school taxi, underneath his swimming things.

I make George's breakfast - but I have to pretend it's not his breakfast. "I'm making this for Sam," I announce, pointedly. I toast two slices of rice bread; Sam's diet excludes wheat, oats, barley, rye and all dairy products. I place them on two plates, which George has selected by sniffing. I spread Marmite in an even layer right up to the edge of the crusts, cut them into quarters, then busy myself elsewhere. George slips down from the Aga; as long as my back's turned, he'll risk the toast. "These are for Sam," he states as he starts to eat. "Yes, they're for Sam," I confirm, without looking round.

Sam's always the last up. He's awake, but he's under his duvet, murmuring; his vocalisation is somewhere between a hum and a chant, and is almost completely incomprehensible. He fingers the toy owl he's had since babyhood. The owl has no name, no character; Sam has never played with him, but then, he's rarely played with any toy.

Sam won't get up and dress until the taxi driver rings the doorbell. I did try ringing it myself, to get him moving, but Sam's not daft. He only fell for that once. And the dressing process can be infuriatingly slow. Pants on - pants off again. Shirt inside out - outside in - inside out once more. Six pairs of identical tracksuit trousers rejected - the seventh finally, mysteriously, acceptable. Socks stuffed down into the toes of his trainers, pulled out, stuffed in again. Endless little rituals. If I try to intervene, the whole process starts all over again. At last he's dressed - no time for niceties like washing or brushing teeth.

The taxi's here. I note with pleasure that the toast has been eaten - but where is George? In the lavatory, of course, where he spends about a quarter of his waking hours. He emerges, and makes for the front door - but wait, there's something odd about his gait. He's pulled up his trousers, but forgotten about his pants. I ignore his protests, hoick up his pants, waft a brush over his uncut hair, and propel him towards the taxi. "Don't wave! Don't say goodbye!" he commands.

A call from the playroom reminds me of the existence of my youngest child. Jake, four, wants his Ready Brek. He chats as he eats; he'd like to meet Gareth Gates, he'd like to be Young Sportsman of the Year. Have I found his reading book? Can he have three kinds of sugar on his cereal? I take Jake to the local primary school, where he is in reception. He greets his friends on the way in, dismisses me with a hug and a kiss. Neither George nor Sam ever embraces me in greeting or salute.

As I leave, I peep through the window. Jake is cross-legged in the middle of the group; he is listening to what the teacher has to say. His hand shoots up. He's right in there, a proper schoolboy, a social animal. He couldn't be more different from his older brothers - but then, Jake's not autistic.

I suppose every child regards its family life as normal. It's only in adulthood that we reassess the things we took for granted. Certainly, nothing in my stable, effortless childhood prefigured the position I would find myself in as an adult - a single mother of three sons, two of them autistic.

I live in the house where I grew up. It's an overgrown Sussex farmhouse with a large, untidy garden. When my grandfather died, my parents moved in, on April 1 1960 - my first birthday. Thirty years later, I moved back there with my husband, Min, and George, then six months old.

The house hasn't changed much since the 1960s; the externals of George and Sam's childhood experiences are similar to mine. They sleep in the room I once shared with my brothers, and the dragons, sea lions and goldfish painted on the walls by my aunts look down on them as they once looked down on us. The kitchen is still dominated by an inefficient Aga, the crumbling garden walls are still there for them to risk their necks, as my brothers and I used to. We walk through the same fields, splash in the same streams. As the boys lie in bed, they hear the sounds I always heard, the creaks and rustlings of an old and draughty house, the train rumbling through the valley, the crooning of wood pigeons in the big yew tree outside.

Yet internally, their childhoods could hardly be more different from mine. There were three of us children. Charles is two years older than me and Rowan two years younger. We were bookish and unsporty; Charles was quite tough and adventurous, Rowan and I were rather weedy. We went to the village primary school and brought friends home to play. When we weren't fighting, we played elaborate, imaginative games based on invented worlds. Rowan and I ruled jointly over the whimsically named Land of Nursery. Charles's country, the United Moore Republic, was more austere. It was a military dictatorship with no female inhabitants.

We moved freely in and out of each other's games, though we bickered as we did so. Play of this kind hardly exists for George and Sam. Their play is limited, end-stopped and - above all - unshared.

As a baby, George was healthy and gained weight fast. He fixed his attention on faces and objects for long periods. He was excited by anything red - and, 13 years later, he still is. He would vibrate with pleasure at the sight of a teddy with a red bow. He didn't reject physical contact, which is often the first sign of autism in babies, but he was never relaxed. He accepted hugs but didn't return them. George wanted to be held up, so that he could see things.

It wasn't possible to think of the infant George as anything other than highly intelligent. Baby books rightly warn us against interpreting the early reaching of milestones as significant, but we can't help it. If a child is showing enthusiasm for picture books at three months, or sitting confidently astride ride-on toys at six months, as George was, adults just do say, "Isn't he bright?"

I am not sure, now, how to interpret George's precocity, which continued until the age of precisely two and a quarter. Does it mean that he really does have great intelligence, which with time has become shrouded in the mists of autism? As I write, 13 years later, George's academic skills are scant. He has basic numeracy and literacy, no more, although he still learns songs quickly and can sing well when he tries.

My own feeling is that George, as a baby, sought stimulus all the time, and at that stage new knowledge and new skills provided stimulus. It doesn't now. George crammed himself with knowledge for two and a quarter years, he overloaded, and he shut down.

George's autism was diagnosed just before his fourth birthday. At that time, as the doctor's notes record, we had "no worries" about two-year-old Sam - indeed, we used some of Sam's abilities, such as pedalling a trike, to highlight George's deficits. It may seem odd, having one autistic child, that we failed to spot the symptoms in our second, but from the start Sam was very, very different from his brother. We got used to the idea that one set of characteristics - George's - amounted to autism, but there seemed to be little overlap between these characteristics and Sam's behaviour, eccentric though that often was. And besides, I'd never heard of more than one autistic child in a family. It didn't occur to me that it could happen. I've since discovered that it isn't even particularly rare.

Sam's apparently cheerful nature also delayed the need to worry about him. George was such a bundle of nervous energy - volatile, ethereal, powerfully attractive. Sam, from birth, was the opposite - placid, quiet, a creature of habit, the kind of baby you have to remember to notice.

Sam passed a number of milestones early, he was strong and muscular, and could sit up sooner than most, but there was nothing like George's accelerated development. Sam smiled, handled toys, looked at pictures. Thinking about cheery 20-month-old Sam, it's easy to see why so many parents believe that they have "lost" a normal child to autism, and why they devote so much time and effort in trying to find that sunny baby again. I don't wish to contradict such parents. I can't rule out the possibility that, in some cases, autism was triggered by some external disaster. But I feel certain that this is not what happened to either George or Sam.

George and Sam were born autistic. It's just that nobody noticed. And nobody can be blamed for not noticing, because the signs were so subtle. People often ask whether receiving the diagnosis was a terrible shock. I can't honestly remember much about my state of mind, but I don't think "shock" is the right word. There was even some element of relief. George, my all-but-four-year-old, was still in nappies and still had the sleep habits of a newborn baby - now I knew this wasn't entirely my fault.

Dr Baird said he had Asperger's syndrome (it was changed to "autism" a couple of years later), and I took her opinion seriously, but I tended to accentuate the positive. "He'll find easy things difficult, and difficult things easy," she said. That didn't sound too bad. We were told that there was no cure, that the right education was the only appropriate treatment. When I left the room, I didn't have the impression that I had a child with a lifelong, all-pervasive disability. I just had George, and now I had an explanation for some of his problems.

We were visited at home by a special-needs nurse a couple of times, but all I remember her saying was that learning to ride could be good for George, and that I shouldn't give him drinks at night. She was more concerned about Min, who had a very bad reaction to the news, and who was in fact about to have a full-scale nervous breakdown.

I'm ashamed to say that I can't recall the exact chain of events leading to Sam's diagnosis. I paid so much less attention to Sam. That is partly because I was already so preoccupied, and partly because Sam has never sought attention. George does, and always has. You can't ignore George, because he doesn't want you to ignore him. Even now, when he repeatedly orders us, "Don't look at me! Don't talk about me!" it feels as if he doesn't exactly mean it. If we truly screen him out, he redirects our attention to himself with ever shriller commands - "Mum, I said don't look at me!" And I wasn't.

Sam has never been like this. Sam doesn't mind whether or not you look at him. Sam is indifferent, whereas George is thin-skinned. Sam will sometimes tell me to "Go a-WAY", and give me a push; this is either because I'm the bearer of bad tidings -eg, it's time to get dressed for school - or because he, Sam, has embarked on an enjoyable course of action, such as emptying shampoo down the drain, or flushing socks down the lavatory, to which he knows I'll put a stop.

So, no, I can't quite remember who first started worrying about Sam. I do remember that he was visited at home by the educational psychologist - perhaps the health visitor had sounded some warning notes. I do remember that he wouldn't cooperate with the activities provided by the ed psych - simple puzzles, handling scissors, building with blocks. "Oh, he can do all this," I assured her, "he just doesn't want to."

"The point is, he should want to," she said, and it was then that I realised something was up.

The educational psychologist, who was very sensible in every other way, said that we didn't need to "push for a label". "Let's just put down 'language delay'," she suggested. I came to disagree. This is an issue I now feel very strongly about. Every late-diagnosed Asperger's or autistic adult I've ever come across has spoken of the relief they felt when their condition was finally identified. Luke Jackson, a teenager with Asperger's, writes eloquently in his excellent book Freaks, Geeks And Asperger Syndrome, "My first reaction was relief. It was as if I had a weight lifted off my shoulders... If anyone is wondering when to tell their child that they have AS, then in my humble opinion, the answer is right now!"

In April 1995, when Sam was four and a half, we were seen by Dr S, a top paediatrician at our local hospital. And there it was. Two very different boys, same diagnosis. But knowledge is power, up to a point. Once you know what you're up against, you can start planning the best ways of dealing with it.

Spring 1999. Sam, my Aunt Meriel and I stood in the cathedral cloister at Hereford. Meriel, being the bishop's wife, lives in the palace; we had left George and Jake under the supervision of the palace staff while we nipped out with Sam for a change of scene. We whirled through the cathedral - Sam's whole-hearted enjoyment of church acoustics keeps such visits brief - and out into the cloister. Sam was with us. We stooped for one second to examine the inscription on a memorial - and Sam was gone.

Sam had often disappeared at home, but I'd worried less because he has an excellent sense of direction. When Sam is "lost" he's not truly lost, just absent without leave. But this time it was different. Sam didn't know Hereford. We were right in the centre of the city. The dangers were real - crowds, traffic, the wide, fast-flowing river Wye.

Calling to an autistic child is pointless, because even if they hear you, they don't usually feel any need to respond. I tend to do it, anyway, because it's what mothers do when children are lost. At home, when we're walking on the farm, I can often locate Sam not because he listens out for me, but because I listen out for him. Sam may not say much, but he's rarely silent. He drones, hums, vocalises and bangs his hands together to the accompaniment of a nasal sound like that of a deep-toned mosquito magnified several thousand times. When he was younger, his noises bore closer relation to "real" sounds. For instance, he used to do a cheerful and accurate imitation of the torrent of bloodcurdling abuse our neighbouring farmer used to hurl at her sheep. He would imitate a drill, a chainsaw, or the final, climactic shudder of the washing machine. Over time, these sounds have become abstracted from their originals, and now form a flow of white noise.

Hereford city centre, however, is crowded with sounds competing for attention. Meriel and I ran around the cathedral precinct for a while, vainly calling, and then I ran back to the palace and rang the police. I went back out to find Meriel. She'd found a young man who had seen Sam heading for the river and had called the police. He hadn't stopped him - but why should he? Sam was seven, strong and agile; it wasn't as if he was a wandering toddler. And if he had got hold of him, what would he do with him? No use asking Sam his name and address. It was good of the young man to do anything at all. "I thought it was a bit unusual," he said, "because he went up to a dog, took the ball out of its mouth, and put it in his own."

We forged on. I can't remember the exact sequence of events, but before long I was touring the city in a police car, eyes peeled. At last, a call came through. A woman had found Sam dancing on the treacherous bank of the Wye. She had taken him to the corner shop at the end of the nearest street. We drove there with all speed. Sam was hopping up and down in the middle of the shop, shouting, "Lollipop!"

Sam was not hurt, neither was he frightened, or even especially bewildered. The whole episode, which had seemed like an eternity to me, had taken, I think, less than an hour, but it would not have made much difference to Sam if he had been away all day. Sam doesn't get frightened because he lacks the capacity to imagine dangers. He doesn't associate me, or adults in general, with security, so he doesn't mind not being with an adult. Sam does have fears, but they are not the kind that would oppress the mind of a normal seven-year-old alone in an unfamiliar city. All children have irrational fears: Jake won't go alone up one of our flights of stairs because he fears that Buzz and Woody (from Toy Story) are waiting at the top. And for a long time he made me come to the lavatory with him because, he said, Zebedee from The Magic Roundabout might boing up out of the pan.

But Jake, alone and lost in Hereford, wouldn't have worried about Buzz or Zebedee. He would have prioritised his fears; his main concern would have been to get back to me, and he would have known that explaining his predicament to another adult would have been the best way to effect this. At present, Jake is worried about kidnappers, because of a note his school sent home about the strange behaviour of an individual seen in the village. He is particularly anxious that Sam might be kidnapped, though somewhat cheered by the thought that anyone who kidnapped Sam might reconsider their chosen career path. Sam has no such worries. Sam's fears would remain approximately the same whether he was careering along through crowds and traffic or safe at home with me.

The Hereford incident was the most worrying, because we were so far from home, but there have been many similar escapes. Police helicopters have been called out on several occasions. Sam's youthful passion for oast houses left him with a dominant ambition - to reach a particular oast that stood on the top of a hill about a mile and a half away. When Sam escaped from home I think he was usually trying to reach this nirvana. Once, he succeeded. Its owner, taking an afternoon nap, was startled to be joined in bed by a small boy still wearing his wellington boots.

It wasn't always Sam. Devon police once retrieved George from a pig field near his grandparents' house - he'd gone out at dusk to have a wallow in the mud. George moves more slowly than Sam, who can outrun the wind, but if he gets an idea into his head, he's hard to stop. I don't doubt that tough, fearless Sam would do pretty well left to his own devices, though the vague, limp-wristed George wouldn't last five minutes. Autists do have sense, of a kind. But "common" sense is closely linked to social sense, and this is something they find almost impossible to acquire.

Reading Hermione Lee's biography of Virginia Woolf, I came across Woolf's callous description of her "backward" half-sister, Laura Stephen. "A vacant-eyed girl," Woolf calls her, "whose idiocy was daily becoming more obvious, who could hardly read, who would throw scissors into the fire, who was tongue-tied and stammered and yet had to appear at table with the rest of us." Laura had odd vocal mannerisms ("a queer squeaking or semi-stammering or spasmodic uttering"). She was "given to uncontrolled gestures like spitting out or choking on her food", and, to the immense frustration of the whole family, had a "strange perversity", which meant she disregarded all their reprimands. Lee raises autism as a possibility; I think it a certainty. Autists ignore requests to stop not because they want to defy you, but because they are socially indifferent.

A high-functioning autist can develop unrivalled expertise in a chosen subject, but it's unlikely that the interest will lead on to exploration of related areas. The autist might know, say, all the butterflies of Europe, but remain indifferent to moths. In Autism: Explaining The Enigma, Uta Frith cites the example of a man who obsessively collected the addresses of juvenile courts. Well-wishers, hoping to expand his horizons, provided him with information about courts of other kinds. The information was rejected. When asked why he didn't want to know the addresses of non-juvenile courts, the autist replied, "They bore me to tears."

Fair enough. When George was about six, he became attached to a poster on the wall of the staff room of his primary school. It was a poster issued by the county council, headed "East Sussex Advisory, Inspection and Training Service" and showed the photographs, names and telephone numbers of 48 council officers. George displayed such a passion for this poster that he was allowed to take it home with him. I still have it. I do not understand wherein its appeal lay, but I'd bet that a similar poster of, say, workers for Connex South East or the Hastings police force would have bored George to tears.

George also had his Thomas the Tank Engine phase. An admiration for the works of Rev W Awdry is almost a diagnostic requirement of autism. It's easy to see why the engines appeal to a systemising brain. They each have a name, a number, a colour (though sometimes, alarmingly, they go for a respray), a similar-but-different shape, a designated function. Their faces are broad and clear, their range of expressions limited and well defined. It's easier for the autistic child to see that Gordon is cross or Percy is mischievous than it is for him to judge the moods of his own mother.

George also latched on to Awdry's characteristic turns of phrase. "'Stow it, you two,' protested Bert." "'I don't know whether I'm standing on my dome or my wheels.' " George used to chant these and other phrases constantly, and they still occasionally surface to this day. He developed a particular liking for one of the engines, but, being George, it couldn't be one of the main players, such as Henry, Edward or Thomas himself. No, it had to be Douglas.

Douglas is Scottish, and his speech is heavily accented ("Then awa' wi'ye, Donal', an' tak' yon guids"), which meant reading his dialogue aloud was jaw-achingly tedious for me. Douglas is also a twin. Donald is No 9 and Douglas is No 10. They are both black, always an important colour for George. Apart from their numbers they are identical, at least as far as I can see. But George only liked Douglas. He wasn't interested in Donald. Donald bored him to tears.

The house is full of small signs of autism. The locked doors and cupboards are the obvious ones, but there are others, if you know what you're looking for. Any framed photographs have been placed face down by George, who finds photographs of people too much for his nerves. A search through the kitchen bin will reveal evidence of Sam - a T-shirt he's ripped up, the chewed-up tyres off one of Jake's toy lorries, a bowl and plate that he's thrown away along with their rejected contents.

Autism directs the way I lead my life, in big things and in small. I say "directs" rather than "dictates". Unlike most parents, I can't foresee a time when my children will be independent of me. I'm fortunate in that, as a writer, I can work from home, and fit my working hours around their timetables. I'm also lucky in that I live in the place I like best.

Moving house would, I imagine, be traumatic for most autists, change-resistant as they are, but I don't want to move, now or ever. My travel opportunities are limited, though I do get my breaks. I took George and Sam to France a couple of times when they were younger, but George really hated being on the plane and the airport was not an ideal environment. "Whose child is that?" I heard someone say, as I struggled with the buckles on Sam's pushchair. I felt I could guess whose child it was. I wheeled round, just in time to see George's feet disappearing through the curtain on the luggage carousel.

The petty restrictions are, if anything, harder to tolerate than the big limitations. I'd like to be able to have a bath without anybody else joining me in it. I'd like to open my handbag without finding a bitten-off lipstick or a capless, leaking pen. I'd like to be able to leave a pot boiling on the stove while I answer the door, without finding that an ingredient I hadn't bargained for has been added in my absence. I'd like to be able to watch television. I'd love to be able to get on with anything - reading, writing, gardening - secure in the knowledge that my sons are all safely and constructively occupied without my constant vigilance. I'd like not to have to load the washing machine twice a day. But this is just how life is, and I don't waste time or energy fretting about it.

Many of the most aggravating habits of normal children are refreshingly absent. The boys have plenty of aggravating habits of their own, of course, but they don't whinge, compete, squabble or blame other people for their own shortcomings. They don't exaggerate minor injuries or try to get someone else into trouble.

From George and Sam I never hear those tiresome phrases like "It's not fair" or "He started it" or "It wasn't me". They may get cross with me when I thwart their desires, but they never criticise me, or anyone else. They never clamour for expensive treats or insist upon their rights; they are unmoved by crazes and playground fashions. They haven't the least notion of "cool". They are immune to peer pressure. They are always completely themselves.

© Charlotte Moore, 2004.