So many theories. I think there are a ton in denial. If you are autistic, with very little support needs, and also no ADHA and weren’t traumatized by society, you could literally function under the radar your whole life, no problem. No one would have ever suspected, not even yourself!
I'm mid-60's. Autism wasn't even a 'thing' back then. Had it been, I may have still gone undiagnosed, but my family and I, and pretty much everyone I met, did know I was somehow 'different'. My diagnosis sure clarified a shitload of things about my past and present life. It was a relief.
I hear you. I’m mid 50’s female. I had ADHD so bad and autism. They called me stupid and traumatized me. I had to teach myself. I managed all the way up to a masters thesis in art history and couldn’t deal with the elitist snobbery anymore. Still can’t. Refused to conform, thumbed my nose at it.