a very good question.
I've read some of the books mentioned, and each time I keep coming back to the C.S. Lewis dictum about books being good books, no matter what age group they're written for. As an adult reader of Children's/Young Adult books, and as a child/young adult reader of adult books, I've never really thought about "age appropriate". If it was(is) a good story, that's all that mattered.
Then, and now.'ve