ADHD is vastly overdiagnosed and many children are just immature, say scientists

This seems consistent with the relative age effect: https://en.wikipedia.org/wiki/Relative_age_effect And matches earlier work done on the same topic: http://www.cmaj.ca/content/184/7/755.abstract

The other side of the coin is that the oldest students in their class often perform the best early on, with a lasting impact on the rest of their academic lives. The effect is also apparent in sports, with children born closer to the cutoff date being substantially over-represented at the professional level.