I cant remember exactly how/when I figured it out, but I went through the bi -> ace pipeline. I was probably 15 or so when I realized I had never been sexually attracted to anyone. I dont think there was any one moment of epiphany, but a buildup of hints that led me to discovering myself.
The only person I interacted with at the time was a total horndog that only talked about sex, and I just never cared about that stuff so when I learned asexuality was a thing it all clicked.

I also think the oversexualization of everything and "sex sells" having absolutely no effect on me helped me to realize I wasnt like others. I couldnt (and still cant) comprehend the grip sexual things have on people.