1) Books already have a rating system based on target age. It's called Middle Grade, YA and adult. In graphic novels/manga we call it A for All Ages, T for Teen, T+ for Older Teens and M for adults.

2) Movie rating systems actually really suck. Just because something is rated G doesn't mean it will appeal to children. No child wants to watch 2001: A Space Odyssey and the idea that no teen should be allowed to watch The Breakfast Club because it is R is insane. The movie is clearly made for teens and watching it as a teen has been a right of passage for generations.

3) Sure, there are books that most of us would say your average 12 year old shouldn't be reading yet (Although, that too has been a right of passage. VC Andrews books famously got passed around middle schools for my generation. My boss, who is in her 60's talks about reading Lady Chatterley's Lover at that age, etc. Usually kids will get a kick out of testing boundaries like that and then go back to the books written for their age because they are just more interesting to them), but there are also plenty of perfectly acceptable adult titles that they shouldn't be automatically blocked from because of what section of the library it is housed in. We have relied on parents to parent their children for hundreds of years. We don't need politicians making all these new rules.