I can't speak for my school friend, but most of the friends I've made as an adult are ND or have ND kids. We were drawn together by similar circumstances, mostly around our kids, but we also just click.
My DC are all Autistic, XH and I both have siblings who are probably autistic, I'm becoming more and more sure I have ADD, maybe Autism as well. Everyone I know with Autistic kids its somewhere in the family, often not diagnosed though. I know several people who have sought and got a diagnosis as an adult after their kids have been diagnosed. I know quite a few people in their late thirties to late forties who are probably Autistic and believe they are, but cant afford to seek a diagnosis or don't see the point of seeking one.
I think Autism and ADHD were massively under diagnosed in previous generations so we can't really say what's behind the increase in diagnoses. Of my 3 Autistic kids only one MIGHT have gotten a diagnosis 20 years ago, the other two certainly wouldn't have. There's no real way to work out historic levels of ND. You could look at current diagnoses in terms of if they'd have been diagnosed 20, 30, 40 years ago. Of the Autistic kids I know Id say it would be only a couple out of 20, 1 in 10 maybe. There's no way to know if that's representative. You could use family histories, but self reporting isnt terribly reliable.
I know a lot of Autistic kids, I know a lot of Autistic adults, but I don't think that's necessarily representive of the world at large. I'm connected to the wider ND community originally because of my kids, more so now because the people I get on best with seem to be ND, maybe mum's of ND kids share certain personality traits because of the genetic causes of Autism and ADHD, maybe we're all ND. I don't know the answer, but it's where i fit in.