I've heard ADHD didn't used to be a problem back when most people were farming and working with their hands. Only in the postindustrial age, when most people in the "developed" world were working office or service jobs, did it get to be an issue. People with ADHD in fact have an easier time sticking to repetitive physical tasks than "neurotypical" people, so they probably did very well as farmers and craftsmen.

The occupations to which most of us apply our minds these days are not things that we as a species are cognitively adapted for. Our brains are optimized for a hunter-gatherer kind of life, not for sitting at a desk typing up quarterly reports. No wonder so many people these days are stressed and depressed.

Regarding girls with ADHD - and Asperger's, for that matter - they're harder to diagnose because, as you said, they don't manifest their conditions in the same way that boys do. This is a cultural thing more than a biological thing. Boys are taught that it's more acceptable for them to be loud and act out, while girls are taught to be more subdued and to interact with the world at large in a different way (linguistic anthropologist Deborah Tannen has done some very interesting work on this).

I agree fully with your last statement - that learning environments designed to engage kids with ADHD would help "neurotypical" kids as well. The lecture method of teaching, which is how teaching is usually done, is actually one of the least effective ways to get anyone to learn anything. Neurotypical people happen to tolerate it better than people with ADHD, but it's still not a great way to teach them.