My daughter was diagnosed at the age of 6 and I was diagnosed not too long after when I was 31. I flew under the radar though multiple teachers and counselors expressed concerns. Not even a trip to the ER raised red flags. I was just a "depressed teen," but it was way more than that.
I completely missed signs in my daughter though in retrospect, there were signs when she was an infant. She was so much like me that I didn't think anything of it. Her kindergarten teacher was the one who raised concerns and I figuratively had to be hit over the head to finally get what the school was hinting at and got her diagnosed. Her behavior is what differentiated us. When she got overstimulated, she acted out. She rocked, she stimmed, she meowed, she ran into things, people, etc. When I got overstimulated, I withdrew into myself. No one questions why the girl in the corner is so shy, quiet, and didn't have any friends. I was a star student, so teachers liked me, but every single one said I needed to speak up more. I was quirky and dressed funny, but that was just put down as awkwardness. If my daughter's behavior wasn't so problematic at school, I wonder if it would have ever been caught. When I brought individual problems to the pediatrician, my concerns would always be brushed off because a lot of it were things NT children can do. She had major sleep problems, she smeared poop everywhere, she didn't toilet train until 4, she always needed to be squeezed, she overreacted, had severe tantrums, ate the same things everyday, didn't respond to her name, had zero interest in other kids, etc. If I put it all together, maybe the pediatrician would've caught it. Maybe.
Hope this wasn't too hard to follow. I try not to beat myself up too much for not recognizing the signs.