Harry Potter has never even heard of Hogwarts when the letters start dropping on the doormat at number four, Privet Drive. Addressed in green ink on yellowish parchment with a purple seal, they are swiftly confiscated by his grisly aunt and uncle. Then, on Harry's eleventh birthday, a great beetle-eyed giant of a man called Rubeus Hagrid bursts in with some astonishing news: Harry Potter is a wizard, and he has a place at Hogwarts School of Witchcraft and Wizardry. An incredible adventure is about to begin!
I was read the first three Harry Potter books as a child, I remember sitting under a camping lamp in a blackout and listening to mum weave the tale of a scruffy, awkward boy discovery his 'weird' was actually his magic. I didn't read the whole series myself until I was in my early twenties, by that time I had seen all the movies and even read fanfiction so I went into it with the expectation of brilliance and I really wasn't disappointed. It's one of those series where the main characters are fine but the depth in the supporting characters and their histories and personality really engulfs you in the world of Harry Potter. Ginny, for example, is probably my favourite character as she comes into herself and grows out of being just the shy girl in the shadow of her brothers and becomes a force to be reckoned with.
I recommend the first Harry Potter book for 9+ every kid is different so if they are sensitive to scary content perhaps save it for 11+ then after that I usually suggest after the third book kids wait till they are a little older maybe 13-14 to read the last three as they do get a lot scarier and more content heavy as the war looms closer and Harry, Ron and Hermione get older. The best reading experience usually happens when you can relate to the characters so matching the reader to the age of Harry (give or take a year or two) is a good guide.
I'll be writing a list of good books to go onto if your reader loved Harry Potter soon so keep an eye on the Middle Fiction Blog Posts.