Thread is a few years old, so don't know if greenlotus' kid has grown, but my youngest is still small and folks think she is younger than she is (turns 12 next month, 4'-6" & 56 lbs). Perhaps the librarian is thinking about content. Even if kids can read and comprehend adult books, some books might not be appropriate.
Middle kid, heading to college in the fall, works at a comic book store. When parents ask about certain comics for their kids, she advises them if she thinks the content is not kid appropriate. In fact, if the kid is gifted, they may understand some very adult concepts, which is even more reason why they shouldn't read certain stuff.
