Despite being a voracious reader all my life, I'm surprised I never noticed until now that there is no literary equivalent of the ESRB. Perhaps someone should work on that.
As far as what rating a book has as to who should be allowed to read it, what do you think? If you are a voracious reader, would you be happy having certain book topics restricted based on your age? Or maybe what your parents thought was right? Maybe they didn't want you know about certain topics. Be careful what you wish for.
When I was a child, I can recall reading 2 books that contained (rather different) graphic accounts of characters coming to significant harm. I absolutely wish they had been better restricted.