Yes and no. I started reading them when I was 10. Enjoyed them, but, now (21) I read them again and I got so much more out of them. Children can read them for the stories, but, adults can read them and understand them so much better than I child would be able to. Young and old can enjoy.
I think that they are books that can entertain anyone 7 and above. I read the first 2 w/ my dad when I was about 6 or 7 but then I read the rest on my own and now I'm nearly 13 and I still love to read them.