My parents' edict was that if I could't understand something in a book, however insignificant, I wasn't old enough for it. I think the only thing they explicitly banned was Lady Chatterley's Lover, when I was 11ish.
Little did they know I had found a book in the fifth-sixth grade cupboard of the school library that had explicit sex on the second page and rather a lot of assault throughout. I remember being very solemn and 9 and telling the librarian to put it in the eleventh-twelfth grade cupboard; I also know she didn't because one of my juniors read it the very next year. *sigh*

Adventure stories with the sex snipped out is much better.