Welp, it's right from the government's "mouth" - 5 on or before Sept 1.Not sure where you live?
Calabasas where they live, schools start Aug 20, San Diego schools start on Sept 3, Aug 12 in L.A. and it's Aug 18th in San Fran.
I think it's pretty much normal across most states for the 5 by Sept rule.
In my area it's more common to "red shirt" kids, boys especially, a year until they're 6. It's hard to sit in full day Kinder for a lot of kids at 5.
I agree with that last sentence because Kindergarten isn't like when we were little. I remember nap time. Now K is like college for toddlers.