DS watched them at 6, all of them, one each day of our holiday. He was fine. He has read the first one but has zero interest in reading the others, and isn't bothered about watching the films if they happen to be on TV.
He's now 8 and loves watching Just Add Magic on Prime 
I can't say I'm a HP fan, the film's are shite and I have also only read the first, back in 2001 at a book club, we all wanted to see what the fuss was.
Anyhoo I digress!