The title is somewhat misleading -- it's not like Kindergarten causes ADHD. The key observation is that if you grab any group of young kids with an age spread of a full year, "younger children may be mistakenly diagnosed as having ADHD, when in fact they are simply less mature." You cannot just pretend they're all at the same stage of development.

You see a similar effect in sports (where again, older kids in the group will have a big advantage).

That's not quite it either. The reason Kindergarten is important is because it's examining kids among their peers. Kids who are on average 6 months younger than their peers are more likely to be diagnosed with ADHD.

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.

Generally kids who are 5 years old (if I'm not mistaken) are in Kindergarten. Some kids start Kindergarten at 4 because their birthday is later in the year. However there is a cut-off date (probably in November or December) and students born after that cut-off date are enrolled in next years Kindergarten class.

So if the cut-off date is December 1st, kids born on December 1st will start Kindergarten a year earlier than kids born on December 2nd.