Depends on the child really. I personally started reading by myself at age 9. And some of those did touch on concepts many would consider inappropriate for my age and reading level. Though none were graphic in detail. My mother believed in instilling culture and knowledge, to be articulate.

I'm no expert. I'm also not a parent though I do have a niece and nephew who are getting into reading. In my mind, the child's interest would play a part in it. And it is up to the adult in their life to have conversations with them. To discuss, explain concepts and answer their questions.