I sorta knew when I was a little kid, to an extent. When I was eight I told my mom I wanted to be a boy. She mentioned that people could get sex changes, and I just said, "NO! I want to have been born as a boy. Otherwise it's just weird." Honestly, the only reason I thought it was "weird" was because I knew I'd get mocked if I said I wanted to become a guy. And I was right, what do you know. Besides, for years I barely even knew FtMs existed.
I dreaded puberty for years. Once I actually started going through puberty, I hated everything about it, and then I REALLY wished I was a guy. I told my parents about it, and they said it was just a phase and that I'd get through it eventually. What a load of BS. From then on it only got worse.
The end of middle school was the most painful, confusing time of my life (I was around thirteen years old then). I started to despise my body even more, and I couldn't stand being a physically weak girl. To compensate for it, I became extremely violent and uncontrollably angry all the time. I wanted to be a man but I couldn't bring myself to admit it.
TL;DR: I guess I always knew to an extent, but didn't admit it until I was fourteen. That's when I stumbled across a webcomic with a transman in it, and I realized how much I could relate to him and it just carried on from there.