It was never a specific moment, or even a full-on realization. I had a vague impression from a pre-teen age that some of my preferences and situations weren't always the same as some of the other kids I knew, but given that I didn't know them all that well (for SOME reason), there was also always the impression that, oh well, maybe it's just that some people are like this and some people are like that, and there are a lot of external factors that shape kids' lives, so maybe it'll be different later on. I got bullied now and then, but lots of kids got that, right? I didn't have any close friends, but that was random, right?
Even decades later, those were pretty much the assumptions floating around in the back of my head, and I honestly didn't really think about it. I 'passed' fairly easily, in that I'd completed school with reasonably good grades, gone to university, gotten a boring whitecollar job and slowly gained promotions over the years, and all those 'normal' things, so if the thought occurred to me at all, it was more along the lines of "Oh well, I guess I'm pretty introverted, and I have hobbies that aren't common locally but there are plenty of people on the internet with the same ones, and my life might not be Hollywood-dramatic but I don't really hate it."
It was only after hitting middle age, and being accidentally prodded into looking into autism one day, that I ever had the thought that all this stuff in my life which I had thought was just... stuff, random rolls of the dice that could happen to anyone, might actually be a single thing - and the thing had a name.
And so I looked into it more, and made some preliminary enquiries to doctors, and got formally diagnosed. And it was only then that I really thought "Well frickity frick-frack, this might be an actual thing I have to think about now."