I knew trans men were a thing three days before my fifteenth birthday after watching the L Word, and realised that was the word for what I was then. Although I always had a vague memory of when I was six and my friend was telling my teacher about her cousin who had transitioned and was a trans man. I didn't know the words for it though, and it was so alien a concept I didn't connect it with me.
I didn't really notice that there was a difference between boys and girls until I was seven, and boys joined my year at school. I went to a very small village school, and there were only two others in my year, all girls, so it was natural for me to play with them more often, I would have done even if I wasn't trans. When I was seven and more people joined, two girls and three boys, I got down and didn't really understand why, and there were arguments between me and my friends because I wanted to play with the boys more than them. When I moved into the next school, which was a lot bigger, I had trouble trying to fit in because I didn't understand the girl stuff, but the boys didn't want a "girl" playing with them, and it caused me a lot of grief and upset. The same thing happened when I switched school again, although I got into even more trouble, as I thought that the only way to be accepted by the boys as one of them was to be stronger than them, and that meant getting into fights.
I never wanted to wear girls' clothes, ever. I fought against my school skirt the whole time, and I couldn't understand why the girls didn't want to join me into pressuring the school into getting rid of that rule. Even when I was a lot younger I would physically fight not to be put into girls' clothes. Whenever I pictured myself in the future it was always wearing a suit, never female clothes, and with short hair. I always thought that would be who I would grow into.
Physically, one of the biggest indicators was when I was going through puberty and was constantly trying to deny it. I was trying to deny my period starting (it was light at first), and I used to stretch out in front of the big mirror at my grandparents every weekend trying to make my chest disappear so that I wouldn't have to admit that it was growing. In the end I managed to convince myself that it was cancer and I was dying, because I just couldn't admit that I was going through female puberty.