Good post, agnurse.
I am no expert, but I also think that as much as there may be those who wrongly self-diagnose themselves and others, there are more who wrongly disparage the fact that people know their own brain patterns and so on. Those people are not experts either, yet assume that their comments about 'everyone is somewhere on a spectrum', and how 'they could just as easily claim to have ADD because they didn't like sitting still at school are no more likely to be correct than are people who live with those traits and find (or don't find) strategies for dealing with them.
My daughter is diagnosed, and also has dyslexia, and I strongly suspect that I have ADD too. Neither she nor I were of the 'racing round the room' type as children - we both have many of the less 'lively' characteristics mentioned by agnurse above, and we both learned how to mask it to fit into a world that is designed for people who are just wired differently.
We also both believe that ADD, like other traits, is 'an explanation, not an excuse' - another popular misconception is that anyone saying they have or suspect they might have ADD or ADHD is trying to play a system. The problem is that there is no system for most - you either pay thousands for a private diagnosis (thus clogging the queue) or wait for years, until it's too late to have any real benefit.
Yes, I am (tentatively) self-diagnosed. I may decide to pay for a diagnosis, but (a) what's the point at my age? Now that there is more information in the public domain instead of in the hands of the gatekeepers, I can read around and add to the strategies I already have in place, and (b) the detractors complain that private assessors are over-diagnosing anyway, so it wouldn't even stop the put-downs.
As far as children and the higher numbers these days goes, I think that the change in systems of education explain a lot. When I was in primary school, we sat in rows in double desks, and learnt in silence, often by rote. These days children sit at tables, wandering around and asking questions as they occur.
Maybe my generation (like others) learned to fit with a system not designed to get the best out of us? I was often accused of laziness (at school and at home) when I can now see that I was anything but. Yes, I did things at the last minute (I still do) but I got them done - often more efficiently than the plodders who had 'a place for everything' and seemed more conscientious.
I don't know - I'm sure there are many degrees and variations, as with so much about human behaviour - but I wish the cynics would recognise that (unless they are qualified ed-psychs etc themselves, of course) they have no more right to stand on the outside and make negative diagnoses for others than those who know what's going on in their own or someone else's lives might be, regardless of how well masked it is.