Yeah, but the Kindergarten cutoff mechanism is irrelevant -- the point is that if you group young kids by some system that's aligned with the calender year rather than their actual age, they're not really peers. You'd get the same effect if you group them by what year they were born in (i.e. used January 1st as the cutoff date).
You are correct that these traits would apply to any arbitrary group of young children with a one year age spread. Kindergarten is an ideal such group, not only because it encompasses a one year spread of young children, but also because it is a group that is being observed for signs of ADHD.
But yeah, I'm not trying to say that the title isn't misleading - there is a correlation between Kindergarten enrollment dates and ADHD diagnosis and that's it. The article makes this pretty clear but the title does not.
