When I was 4, my first year of school, the teacher told all the girls to gather in the front of the room, they had a special guest, and told all the boys to go to the back and find something to occupy themselves with. I instinctively felt I belonged with the girls, quietly placed myself at the back of the girl-space, and listened to the visitor-- a woman from India demonstrating how to wrap a sari. (Now I give sari-wearing lessons to my friends.) This is the most vivid of many cross-gender memories I have from the first time I became aware of gender differentiation--when I started school. Once I learned that girls and boys were treated as separate categories, I instinctively felt I belonged with the girls. I kept going to the girls' bathroom at my kindergarten, and refused to go to the boys' room no matter how much they scolded and threatened me.
When I was little and all my cousins gathered, all the girls would sequester themselves in my sister's or a girl cousin's room, shut the door, and have a session of girl talk. Meanwhile, all the boy cousins would be playing football or shooting baskets outside. Again, I felt that inexorable pull that the girl-space was the only place I belonged. At the time I never analyzed it or explained it, I just felt it instinctively. My childhood gender trauma is epitomized in that memory of me sitting alone on the floor outside the locked door of the girls' room, desperately wishing for some way to be admitted to their social group.
Early in grade school, I spent my recess playing hopscotch with the girls-- until the powers that be forced me to leave there and join the boys in their rough and tumble ball games, which I hated. As always, I wound up isolated and alone. And bullied by some of the boys. I could give many other examples of how I was refused admission to female society and felt abhorrence toward male society. So I had neither.
Thing is, at the time I never clearly articulated to myself what my problem was. Instead, I just internalized the harsh messages from my parents, teachers, and other kids that there was something wrong with me. I guess I just assumed I was a misfit for unknown reasons. Throughout my life, being forced into all-male socialization was a living hell. At times I wanted to die. I made friendships with girls as much as possible, though the all-male circumstances I was often forced into made that challenging. I had almost no friends.
I went through my life feeling I needed to be a woman. And yet, I never faced up to being trans. From what little I knew of it, the concept seemed so utterly exotic, outlandish, and frightening that I didn't think it could apply to me. When I was 22 I met another trans woman for the first time, and that brought me to realize that I really wanted to do what she was doing.
So I guess the answer is age 22. Except that the poor lady I met at that age did not pass well at all. From that I got a negative impression of my chances. I buried the thought again. But it kept coming up again, though I kept denying it. I should have been a woman. Nah, it's impossible, never gonna happen, forget it. I should have been a woman. Pesky, persistent voice of my inner self.
The power of denial was so strong that I remember thinking to myself when I was 44: Yeah I should have been born with a vagina, but no I'm not trans. As stupid as that sounds. When I was 43 I first heard a scholar presenting a paper at a conference give a definition of transgender, and with a shock of recognition I realized that the weird thing that had been in me all along had a name. But I still kept denying that it really applied to me, and it took until I was 45 before I finally dropped the denial, admitted I'm trans, began consciously dealing with it.
So depending on how the question is interpreted, the answer could be age 4, 22, 43, or 45. Take your pick.