How about common sense and letting parents, teachers, PTAs, and trained librarians decide what is age appropriate for the kids?
If the kid wants a book that isn't in the library, or the parents decide it's time to teach the yard ape how to make a bomb, it's up to the kid to ask the parents, and the parents to do the teaching.
Common sense can work really well if we let it. There are few truly dangerous ideas. There is, however, a whole boatload of truly dangerous ignorance.
Pangloss