The mature comic takes roughly the same guidelines as it does for the movies: no reading unless fifteen years or over and accompanied by an adult.
The distinction is important for horror as the bloody excesses of EC inspire black and white depictions for an older readership in the form of Creepy, Eerie, and Vampirella. But the science fiction and fantasy communities have a use for the mature readers tag too as is evidenced by the French comics magazine Metal Hurlant and its English language equivalent Heavy Metal
Mature is not quite the same thing as adult and so we see nudity but not with pubic hair and orifices, coarse language but of a certain stripe and not constant, violence more muted than R but harsh enough to give you the picture.
An extra step in differentiating comics for the mature reader is to not keep them in comic form but create them as a graphic novel or have them bound as a trade paperback. Or you can just offer them for sale in a special section of the shop.