When I talk to therapists or curious cispeople, I always want to say around 3-4 like I read a lot of people saying, but I look back and don't remember having any feelings about gender that young. I was just a kid really, not identifying with male, female, or anything. I remember not always liking the clothes my mom picked out of me; they were often "cute" and "girly".
It wasn't until about 8-or-9-years-old that I realized something was different about me. Most of my friends were girls, but I felt more drawn to boys and copied their behavior. My closest friend was a boy. I got into fights on the playground. I felt weird and different from my classmates. My shoulders were too broad, I was too tall (at the time), I wanted to wear boys clothes. But I never told anyone.
For years, I kept my feelings to myself. I was uncomfortable in my own skin. Late elementary and middle school, I dressed as a boy in my room, putting my hair under a baseball cap and packing with a sock. I used an ace bandage sometimes to try to flatten my chest. I was never quite satisfied with that.
I came out as a lesbian in early high school. This gave me more freedom, I thought, to be masculine. With more exposure to the community, I learned what it meant to be transgender and realized that I was actually a guy this whole time. I was 15 when I started therapy. I'm 20 now and on T.