Being a kid = ADHD.
Parents need to stop resorting to "parenting in a pill" for their own inadequacies, or pressure by society to "act" on their child being precisely that.
That's not to say that ADHD doesn't exist or some-such nonsense, but I believe this study at least is going somewhat in the right direction to prevent overzealous diagnosis.
As someone already stated (and with whom I definitely agree) - the test set is already biased for a large percentage based on age.
Not to mention the pressure that is allowed by teachers. I myself was suddenly diagnosed after ONE of my teachers decided I was too active and made an enormous fuss about it. Took the meds for a couple of weeks, lost ~10 pounds (I'm already a stick), and told them to bugger off.