I'm in my thirties, and i'm still in the process of figuring it all out (we didn't have the same resources and knowlegable adults when I was little), BUT...
Kindergarden I told the teacher all the letter and number stuff was easy (loudly) and she shot me a look I will never forget. I never mentioned it again... hated that teacher.
The first day of school I stood 20 feet away from the jungle gym and stared at the mass of kids playing. I had no idea what to do. My older brother's friend asked what was wrong with me and said I looked stoned (had no idea what he meant). Later I understood the jungle gym and enjoyed it.
In first grade I realized that although math was simple (theoretically) I had trouble conceptualizing the numbers into what they were supposed to be doing (eventually I developed some other way to think about it, I mean, some instictive way). I also realized that I could visualize an image and draw it, nothing terribly complex, but I new this was different, I'm not sure why, but it was something I thought was special. One girl decided I was her boyfriend because I could draw her a dinosaur on command much better than the other boys. In first grade I recall drawing complex steel bridges (stick figures really), with cross bracing and everything, over and over again; they were hung on the wall in class. My best friend turned out to be mildly ret*d and by the end of the year I was pretty unpopular. I examined rocks and sand in the playground, so that was fine.
I have clear memories of being 2 years old which for people my age is apparently odd. The memories are clear (my brother convincing me to eat food on the floor with ants on it, didn't think anything was odd about that at the time).
When I was six I remember staring at a tree in the neighbors backyard and having this really interesting thought that that tree represented the development of human society over the millenia... same year being convinces, for a short time, that the rain I saw outside from my deck would eventually cause the world to flood; this was quite distressing, until after thinking it over and staring and staring for a long time it dawned on me that the water went into the ground, or the streems, and that they ended in the lake, and that clouds were overhead and that water must have come from somewhere, and I though about steam I saw evaporating from a desert lake and likened it to clouds...
I didn't know I was different at a really young age because there was always one or two kids I could seem to pull into my world...
I spent most of my life trying to be the same, and in a way dumb, I think I might need to rethink things now...
Several years ago I thought I might be slightly autistic (in my twenties) because I somehow identified with the classic autistics I saw on TV... It was also one of the first things I told my wife who I met when I was thirty ("I think I might be slightly Autistic"), though she and I thought I was sort of joking ( a day later after ordering pizza I positioned myself on my bed to wait for the bell to ring until I realized this was odd, to which I said to her "see what I mean")... Now I am just waiting to see if I can get a dx.