I think the consensus is that we are born trans, but it takes a while to figure it out! That brain variation happens during foetal development, so we are hard wired from then! There are lots of trans people who never figure out that they are trans, or they know deep down, and will never accept it. Realising you are trans doesn't necessarily mean your life will get better. I knew from the time I discovered boys and girls are different, but it just meant that I had to live with that incongruence dominating my life as I couldn't do anything about it. I sort of envy those who don't find out until later in life, living most of their lives in blissful ignorance.
Hugs,
Allie