I've read the entire thread and thought about this quite a bit. In my childhood, I had parents who were not paying attention most of the time, so I read whatever I could get to. But it was usually only reader's digest condensed books my parents had. We went to the library and they let me get books from the adult side as long as they were ok with mom, which was usually no big deal as I was never choosing anything racy or "bad." But I started reading at 3 and was always reading 3-5 levels above. So I was sometimes reading things that were somewhat too graphic in the scary/gory realm or occasionally in the sexy realm when my parents didn't know. I never said a word of course, but I had nightmares over Jaws for years.
As a parent, I'm a firm believer in controlling what your children see on TV/Movies and in the Popular Music they listen to, News and Radio stations and such. Now don't get me wrong, I don't mean unnecessarily sheltering your children. I'm talking about keeping things age-appropriate for talking points and discussions and such. However, that's talking about things in general. When talking about personal reading material and such, its going to be a child by child basis.
With my oldest who has Asperger's, she is very academic and intellectual about everything. So, I didn't need to regulate her so much. My 8 yr old super ADHD-precocious boy is super impressionable and very hard to change his mind once he's got an idea in his mind. So, I need to be on top of what he's reading. Plus he tends to asks questions til the end of time, even when it is not appropriate for him to be told everything yet. I'm not saying I'll always keep an eye on him. Just for a while. It all depends on maturity level and and social awareness. I can't wait until I don't have to regulate at all and we can all have wonderful intelligent conversations about everything!