When I walk into the library a block from my house I notice one thing in particular: the children's books are at one end of the building and, not surprisingly, the adult books are at the other end of the building. I know. Amazing, right?
The kids' section is made up of smaller sections . . . based on children's ages and reading abilities. The call numbers will begin with a J for Junior: JP (Junior Picture Book), JF (Junior Fiction), etc. As the age and the reading level of the child increases, he or she will migrate to books with a Y (Young Adult) call number: YF (Young Adult Fiction), YNF (Young Adult Non-fiction). (Squidoo has a good webpage for explaining the basics of finding a book in the library at http://www.squidoo.com/bookfinder.)
Adult books are generally classified as fiction or non-fiction. Adult fiction will generally begin with an F, followed by part of the author's last name. If it's genre fiction, the library catalog will indicate the collection where it is shelved: mystery, fantasy, romance, western, and so forth.
Adult non-fiction usually has a call number and part of the author's last name.
Everyone knows this stuff. I'm not mentioning it because I think people don't know their way around a library. I mention it because BOOKS DON'T COME WITH MPAA RATINGS, like movies do. (Even TV and music have a ratings system.)
When you let your child loose in the library (at any age), or when you yourself roam the stacks of books, remember that, beyond the call number classification, books don't come with a ratings system. And especially remember this: books located in the adult books section of the library are ADULT BOOKS. That doesn't mean their content may not be suitable for younger readers, only that you shouldn't ever assume that it is. Those books weren't necessarily written with a young reader—and a young reader's sensibilities—in mind.
And always, always, look for appropriateness if the book your child or teen picks up is from the adult stacks. That's my parental two cents worth, anyway.
(I've included the link to the MPAA ratings web page below, if you're curious to see what thematic elements and contents generate which ratings in movies.)