Not sure why. I think it's because it's impossible to show something "mature" in a movie without it looking cool.

In a book, sure there might be romance, or horror, or death, but it's almost always treated with more substance.

Plus, reading involves more thought, more attention span, and requires actively reading to consume it...

The main reason we consider certain things inappropriate for kids is that we think they can't handle it- maybe they'd get scared, or confused, or too excited and try to imitate it, etc.

But in a book, if they're confused, they'll likely stop reading. Ditto if they're too scared. And I don't know, I'm less worried about a kid acting something out in a book than I am them acting out a wrestling move or whatever.

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It's not the most well put-together theory, but that's basically why I don't really care what kids read. I figure that if they want to read it, and are able to read it, they're generally "ready" to read it.