I've read the responses to this and, well...I realize that a) writing about sex doesn't equal having sex and b) children are exposed to a lot of graphic violence and sex, and that it's silly and naive to imagine they don't know anything about either of them.

However, I think what the OP might have meant was that it's troubling that a 12 year old would know enough about sex to write it convincingly, especially if it involved young characters. Had I read the fic/s in question, it might have set off potential "abuse victim" warning bells.