The way I saw it was that I could read it to him any time, but he would only have had the most basic understanding of it. Oh, right, yeah, boy went to a magic school, think he wore glasses, the end. 80% would go over his head.
So I waited until I could be sure at least a bit more would sink in and he'd actually get some enjoyment from the tale.
I also didn't want him to read the later books too soon again, not just due to the deaths and scariness, but missing most of plot and its nuances.
Anyway. He's 8. At late 6 or 7, I read him 1 and 2, waited a while, then 3. We read 4 together because he begged, this year. I'm leaving it there for a while, mainly because 5 is a bit of a slog to be honest.
He's now reading 1 alone, with the illustrated version, and I'm thinking he can read the first four again this way. Then he'll be, what, 10, 11? by the time he wants to perhaps keep on reading and tackle 5 onwards.
There's only one chance to hear those twists and spoilers the first time, so I'd like it to be at an age where he gasps and he remembers it as I did, rather than be 5 or 6 and just go 'what? Who did what? What's a wand?'