Let me begin by saying that I don't support age rating of books (i.e., saying "this is for ages 8-12, this is for young adults, this is for adults..."). However, when you look at how we rate our other content by age, it seems hypocritical, does it not?
Games and movies receive official ratings that state whether or not the content of those products is suitable for a certain audience. Sometimes, the law enforces these ratings. That means if you're under 18, you can't get into an R-rated movie (without an adult). But you can go and buy a book that may have the same graphic scenes as an R-rated movie, and the cashier at the store doesn't stop you. They don't card you. (At least, they didn't card me when I was under 18.)
Seems like we have a double standard here. I know, I know: books aren't as "visual" as movies or games. Reading about mass violence or sexuality, reading a curse word, that isn't the same as seeing and hearing it. Well I think that insults the average reader's imagination. And even if it doesn't compare to graphical depictions, wouldn't a book's descriptions, if done well enough, still be specific enough to scar immature readers? Should we continue to let unrated books be sold?! Where are you torches and pitchforks, people?
Luckily, since most children and young adults these days aren't interested in reading for pleasure, we don't have to worry about this crisis. :D