I am someone who figured it out all on my own, if it helps,
But like people are saying, you don't just know the definition of what Autism is without being taught; so you have to encounter the information somewhere, and that usually triggers a flash of insight.
Although, a couple years ago, I had a conversation with a girl on OkCupid who I almost dated, who said she thought maybe I was an Aspie (which was a positive for her actually). But I didn't think much of it at the time, and forgot all about it. Probably because we never talked about it as a potentially negative thing that could be having a noticeable impact on me. So to me it was more of a minor curiosity like being double-jointed. You just kind of go, "oh wow, neat" and then it's out of sight, out of mind.
So basically, when I figured it out just this year, it was like I was starting fresh with virtually no prior knowledge.