There isn’t a definitive reason it has turned out this way so far. It could just be coincidence, of course.
The book speculated that it probably doesn’t hurt to hang out with an older crowd when you’re very young, and have to try to keep up. Being older and hanging out with a crowd your own age and younger probably doesn’t help, as far as giving you an incentive to be speedy.
I’m not saying it’s right, or that the pattern is even statistically significant. It was in the book I was reading, and I thought I’d share.