Hmmm... different perspectives and different cultures, I guess, but personally it's the skills of JKR in weaving some of the nastier realities of life into her fairy tale universe (abuse, deception of children by adults, death, loss, war, bullying, low self-esteem, intolerance/xenophobia, questionning beliefs systems the adults impose on you because some adults and authority figures really are prejudiced, bigots or stupid etc.) which makes Harry Potter so valuable (ie: formative) for fairly young children. For the younger ones, that's definitely books to read with them or to them, and I would definitely read the books first to be able to answer questions and discuss topics at need and will with the kid, but there's nothing in there I find too shocking or harsh not to recommend it to 9-10 years if they have the reading skills to go beyond books 1-2-3. It's more a matter of things getting too complex for very young ones, but then that's what re reading a few years later and growing up with a book or series is for.

Fiction in my view is a most excellent way to learn gradually to cope with life and the harsher facts of life. You never know when a child will cross paths with bad adults or experience cruel loss, and in my view the sooner they begin to learn about such things the sooner they get equipped at their level and withing their limits to deal with it. It's how I learned to cope with my childhood fears, reading Stephen King or even Tolkien (and mom had given me the books on purpose). I knew if I admitted the nightmares (damn Gollum and Nazguls!) my parents would take back the books and I'd never know the ending, so I learned to cope with them on my own without waking my parents in the middle of the night anymore.

For 12 y.o. and beyond, in my view, the first books don't have as much to offer about life that they're not already familiar with (or should be) - because Rowling wrote each book with children of Harry's age in mind, so they become little more than entertainment for teenagers and adults. Sooner than later, everyone crosses paths with the Dudleys, Malfoys and co.

Mind you, JKR's books offers a lot of values associated with the left - especially the social, moral and religious left more than the political, so I don't judge either parents leaning on the moral or religious right who find Harry to be conflicting quite a bit with the values they're trying to teach their children.