I believe that especially in the US there might really be
  more people with ADHD brains (in the end leaving your 
  continent to settle on a new one is still perceived as 
  something bold to do in 2014 - think of how much more 
  risky this must have felt over the first couple of 
  centuries after the discovery of America)

I'd never thought of that before. Another possible reason for increased prevalence in the US is that we work more hours and have less vacation time than a lot of European countries, increasing the demands on our focus.

Also, just for solidarity's sake: I was also diagnosed in my 30s, and was called "lazy" (etc.) a lot when young. Bet we went through a lot of the same things.