I think I got lucky. Late July with a school that (at the time) had a Sept 1 cutoff. There was only one person younger than me until middle school.
My kindergarten teacher, who was also a family friend, requested I do it again because I wasn't mature enough. And she was right, honestly; I'd fight against nap time, and remember no end to problems in first grade with sitting still.
However, while maturity played a roll, the problem was I was bored. I went in to kindergarten being able to add, subtract and read, and had already started doing multiplication with my mom (herself a high school teacher). Another year in kindergarten would've been torture for me. Thankfully I had an amazing first grade teacher who recognised the problem. Instead of insisting on ADHD, she gave me more challenging stuff and let me work on my own. She never called on me to read aloud, because she gave me other books to read behind the class's book. This lasted until my 4th grade year, where one of my teachers didn't like it. Thankfully by then I was mature enough to recognise what I needed to do, etc.
So really, I wonder how much of this is a mixture between multiple things and how many times kids get given ADHD diagnoses when what they really need is differentiated education. Granted, this is difficult; I was a math/science secondary school teacher (and I think it's even harder for elementary school teachers, for various reasons ranging from standards to their own competency in the subjects they teach), and it was almost impossible at times.
That said, maybe I do have ADHD. I went on an amphetamine for weight loss at one point, and there were times I noticed I could focus longer and easier. But I've always had a tendency to get bored of something easily, and I'm sure the Internet hasn't made it any better. So I avoid getting tested as it doesn't impact work or study or anything for me (anymore than the Internet itself does)