We've had three kids go through HP.
Our oldest read the first book in first grade and was fine until she saw the movie. Dad told her to hide behind the chair during V drinking the unicorn's blood scene but DD13 recently fessed up that she peeked from behind the chair. That scene gave her nightmares for a week and turned her off of the series until the summer before 6th grade. She then read the whole series in about a week because the last movie was coming out and she wanted to see it with her friends.
DD11 started to read them last summer. She only got through #5. She is my highly empathetic kid. Harry being tortured by a teacher, his general teen angst and an important character death turned her off to any more. She felt Harry's pain and didn't want to do that to herself anymore.
DS8 read them all this past year in second grade. He did fine on his own through #4. With #5, he wanted me to read with him, trade off chapters etc. I think that it was his way of getting emotional support through the darker aspects of the later books. Still, he was determined to get through them and enjoyed them a lot at this age.
