I used to sneak my mom's copy of Stephen King's "It" off the shelf and read it a few pages at a time, despite the fact that I was told not to. I was 9. It didn't necessarily change my view of the world, but it certainly scared the pants off me and gave me nightmares....which, of course, I couldn't mention to my mom, for obvious reasons.
Seriously, though, I think I was 7 or 8 when I read "Across Five Aprils." Cried through most of the book and it gave me a deeper understanding about the real impact of war on a society. I wonder if that was why I totally freaked out in sixth grade when the Gulf War began.
I also read "The Hunchback of Notre Dame" at around ten. Made me rethink my habit of standing by and allowing certain kids at school to be picked on mercilessly.
I don't think I read either of these too early. (The Stephen King was probably a bit early - but that's another story). I think when my kids start asking to read these books, I'll probably let them read almost anything unless it has graphic sex scenes or something. If they're like I was, they'll back off from that type of book if it's too much for them.