Well, I guess I'm in the minority, but I knew that the series got progressively darker so I made a conscious decision not to do them as a read aloud with dd1. Dd1 (9) checked out the first one from school this year to read on her own. A lot of the other kids at school were really into them and reading them (further along) and she felt inspired to give it a go. I was really thrilled because while I wouldn't characterize her as a "reluctant reader" she's been much more comfortable with magazines and shorter books, and graphic novels. I found that very frustrating because this is a kid who lives for stories. I knew she'd be a total bookworm if she could just get started, so Harry Potter was what did the trick.
Â
She's very sensitive, though, so I was a little concerned about that. She's on the 4th one now, though, and the girls at school have told her all about who dies and who ends up with who and lots of other spoilers, which might drive me crazy, but seems to help her feel comfortable with it all and part of the crowd, too. Once dd1 got started reading them, I did go ahead and read them all myself so I knew what she was getting into (I got totally sucked in, too).
Â
Now, they're a little beyond dd2's (7) reading comfort level although she can read them and has started #1, but since they were eager to see the movie for #1 we started doing them as a family read aloud, so they could see the movie. We're on #3 now and they've been fine with it all and with the movies for #1 and #2. I didn't want dd2 to be freaked out and thought knowing the story she'd be okay with the movies. Movies tend to be scarier for my kids.
Â
Dd1 is completely enamored of all things Harry Potter now so I'm really glad we waited. Dd2 likes it, but she's not ga-ga over it like dd1 and I think dd1 is just at the right age/stage for it. I think if I'd jumped the gun with her and read them to her earlier it wouldn't have been as special to her. It's her thing now and the kids at school are really into it, too. I think she really wishes she went to Hogwarts instead!
Â
So, IME, waiting until the kid is ready to read it themselves is a good way to to decide when is the right age.