As I get older, I'm starting to understand the point of age ratings on movies and games. I used to think that when reading something bothered me, that it was intrinsically worrying - not that maybe I was too young to handle it. Now, looking back, I think I was.
I'm thinking mostly of Virginia Andrews' novels. Many of her plots focused on rape/incest - not really the kind of thing a ten-year-old should read. They really disturbed me when I first read them, but going back to them now, they don't bother me so much. I appear to have become hardened in my old age.
Rape/incest is an extreme example, but even lighter things used to bother me, when I was young. Books like Judy Bloome's Forever, which describes a first relationship. And then she cheats on him, and they break up. I didn't like it. First loves were supposed to last.