I guess I'm a freak 'cause I certainly did not know when I was very young. I was quite the tomboy, and everyone said so, so I figured that's what I was. I was fine being a tomboy, as it gave me a special kind of pass. "Oh, she can climb trees with her friends 'cause she's a TOMBOY." "We don't have to worry about her swimming a mile into the ocean--she's a TOMBOY!!!!!" I figured my mom knew my friends would take care of me, as I was (understandably) the runt, and take care of me they did, though reluctantly.
The first time I realized something was wrong, my friend wanted to play some game that required a penis (I think it had to do with erections--to this day I really have no clue). He kind of started and then frowned and said I needed one or I couldn't play. He wasn't mean about it, even trying to make one out of paper for me (lol!) but I felt really bad and refused. The only other time something like that happened was when his mom said we could play in the bath tub together, and I assumed we'd have something on, and when I figured out I'd have to strip I balked. Not because he was a boy and I was a girl, but because I was embarrassed and didn't want him looking at me. I felt inferior.
I identified strongly with the LGB scene at my middle school and started telling people I was bi then. I wore lots of skirts and girly clothes in middle school because that was how the cool girls dressed, and I thought I was a girl.
The first time I talked to someone about being trans was actually the first time I talked to my now best friend online. I didn't even have a word for it ("I feel like I want to be with a boy but not as a girl, as another boy."). I started questioning then, but not too deeply, as I had an abusive boyfriend who would have freaked.
When I finally got away from him I tried to girl thing again, as he hadn't let me wear any girly clothes in a long time. One day it came to me that I wanted to try to "go as a boy", so... I went to the store, bought some Ace bandages, and never looked back.