StoryForge, I won't include any spoilers, but there may be a strong hint or two as to what to expect.
I think the aging process of the protagonists is a key theme of the Harry Potter series.
In C.S. Lewis' Narnia books, we get glimpses of the Pevensie children, Eustace Scrubb, Jill Pole, Digory Kirke, Polly Plummer, Shasta/Cor and Aravis at a certain age. They are all roughly between the ages of seven (Lucy in Wardrobe) and thirteen (Peter in Caspian and Shasta and Aravis in Horse's Boy). While the time period (about seven years) is comparable to the age differences from beginning to end, of Harry, Hermione, and Ron, I don't feel the changes nor challenges from six to thirteen are as great as from eleven to seventeen. Lewis deals with the gamut of childhood through different characters, but he never shows one character moving toward adulthood. We're left to project Lucy's development by reading about Jill, Polly, and Aravis. On the other hand, Rowling takes each character though the aging process... and we get to see how Hermione, Cho, Ginny, Fleur, and Luna develop individually.
I'm not saying that Potter is better than Narnia because of this. Heavens, no! But I am saying that it's a different perspective from what I'd become accustomed to...
Roald Dahl was mentioned above. I only read about Charlie, Danny, and James when I was a kid, but it seems that Dahl's heroes are also set at a certain point in time. Danny saves his father's livlihood, James escapes slavery, and Charlie saves his family (plus the entire Earth)... and they all do this before they reach adolescence. We never learn if James became as nasty as his aunts, if Danny became a professional thief, or if Charlie went on to oppress the Oompa-loompas.
If Rowling had stopped with the Philosopher's Stone/Sorcerer's Stone, then we'd just see Ron as Harry's loyal and dependable sidekick. We'd never know how petty, how jealous, how negative, how self-justifying, how stupid, and how two-faced Ron can be. Then again, we'd never know how gallant and how encouraging Ron can be.
I felt that Rowling really tried, especially through Ron, Draco, Luna, Neville, and Ginny, to show the ups and downs of the overall physiological and psychological changes puberty brings about.
I'm not saying the Potter series is a model guide for surviving adolescence. But Rowling tried to let kids know that you can survive family spats, romantic heartbreaks, sports injuries, and even betrayals. It may take years to overcome emotional and physical scarring, but it can be done. She tries to let kids know that failure is not permanent. A mistake at age twelve does not define you for the next sixty years. And I think this is part of the beauty of the aging process that we get to witness.
Now as to how to give this series to a prepubescent child? I don't have a definitive answer. I'm going to try and give it out piecemeal... one every six to nine months when my niece turns ten. She's already read James and the Giant Peach. I am planning on giving her the entire Narnia series soon... on her ninth birthday. I think she can read the entire series fairly quickly without the pitfalls that paranoid marvin mentioned earlier.
There is a window for being the right age and finding the magic within a story. Ages seven to nine will find the magic in Charlie's, Danny's, and James' stories. Ages eight to ten will find it in Narnia. But the window for finding the magic of Harry Potter is quite a bit longer.... ages ten to seventeen.