I think my oldest was reading George R. R. Martin by age twelve (but I could be a year or two off), on the recommendation of a friend who is somewhat younger, and my second son, who is only just now a teenager, has read some of those books. I have not read anything by him.

On my part, I read the nonfiction book Treblinka by Jean-Francois Steiner

http://www.amazon.com/Treblinka-Jean-Francois-Steiner/dp/045...

by no older than age ten, and no one would describe that as children's literature. (I reread parts of it a few years ago just to check if what I remembered about how graphic the violence in it is was accurate--yes, that is a true book with very grim details about a Nazi death camp.) Children deal with book content on their own terms. I didn't finish Orwell's 1984, which my dad had in the house, until my third attempt (thirteen-going-on-fourteen), because I found that very disturbing, and I still found 1984 disturbing when I reread it last year. Some great literature just isn't child-friendly, fiction or nonfiction, but some of it is well worth reading even during childhood.