I had some indicators when I was young, I always knew I was not like other kids, one of my earliest memories was walking to grade school, getting to the yard outside. The girl I had walked there with went to talk/play with her friends and I would have loved to join them but its grade school in the late 80's the divide between genders is horribly strong just going near her and her friends is out of the question, and I had no desire to play with the boys so I stood by myself alone, something that mostly continued forever.
It was around 16 or so and getting access to the internet that I was finally able to put a name to how I felt and see that I was not crazy, I was still unsure where I stood on transitioning but by 18 or so I knew that if I ever wanted to be happy I would have to transition, so instead I torchered myself for the next 15 years between family and fear and self loathing before I finally took action.
But I agree with some others due to how much more acceptable girls acting rough and tumble is compared to boys acting feminine its understandable that many transmen may end up being older before they realize why they are like they are.
--Serena