I was born 23 years after you. I remember, when I was four, there was a show on that featured Buffy and Jody. I didn't understand that Buffy was a little girl and my mother used to say, "Peter, you can't be like Buffy."
A former housemate of mine had a son who was about two. He used to dress up as Peter Pan, a pirate and he also used to dress up as Dolly Cowgirl. I would say, at that age, he's just a little kid, leave him be. And if I had been an impartial observer of my childhood, I would have said to my mother, "Okay, he might be like a character, but leave him be. It might be a phase, it might not be."
I don't think we should get too concerned with finding the exact cause. What I think we should do is, accept that there are transgender folk, observe and then support them to be who they want to be.