We're twins! I'm almost 53, and I have been realizing I'm autistic over the last few years as well. My sons are 20 and nearly 23. My 23-year-old got the worst of it, since we had no clue what was going on.
Like Dav Pikley, he started to seem sad when he went to school. He most likely has dysgraphia, but he never received any help for it. If you read "Zero Tolerance Kindergarten" you'll get a little taste of what school was like for him.
We did some homeschooling, but I didn't have support from my spouse. I also didn't know much about the possibilities, like unschooling. I agree now that it's the best thing for neurodiverse kids, and probably most kids.
I was a public school teacher and an expert in positive discipline - no rewards or punishments, just compassionate problem solving. Everyone was telling me this was why my child "did not know how to behave." (Ironically, all of those techniques were praised with my students and my second child, who is polite and well-behaved.)
I was so confused I just started doing the opposite of what I believed. He got worse. I finally found online communities and research supporting what I wanted to do all along, but he had a lot of trauma by then. I let him drop out of high school - he was in day class and not learning anything anyway. I urged everyone to let him do "nothing." Two years later he wanted to take the GED - he could have taken the easier CHSPE, but he wanted to prove to everyone that he isn't dumb. He passed, of course.
He is in his own place as of this year. Decided to go off all meds. He felt like the meds were blocking his thinking in some way. Rough for awhile, a big outpouring of emotion, but now he seems good. He is starting entrepreneurial projects and some volunteering. His affect in these settings is so professional and impressive. He gave an online training to a team I work with!
I think I did the right think in letting him take time to heal. I kept having lunch with him, asking if he needed anything, etc., but not forcing anything on him. When he was ready, he was ready.