I'm not autistic, but uh, I can see it from my house. Autism runs strongly in my family (enough so that our genetics have been studied). And I happened to marry an autistic woman. She was undiagnosed at the time - we only learned after getting our son diagnosed a couple years ago.

Since we started learning some of the less common symptoms of autism, we've started noticing them in family members on both sides. Pretty sure both of our fathers are autistic, for instance.

It's just that... If you're "passing" for NT, and/or of an older generation, you probably won't seek out a diagnosis. Remember, autism was only described a couple decades back. As diagnoses go, it's still pretty young - we're still learning a lot about it. Including how prevalent it is.