It definitely seems to be different for everyone, wonderful question to pose, though, Death!
I was aware of crossdressing by like age 13, but I did not nor ever considered crossdressing myself.
I discovered what transgender was around age 22, but again, I never expected that I could or would be transgender.
There are numerous little hints throughout my life, but as I state in my blog thread here:
https://www.susans.org/forums/index.php/topic,237152.msg2131598.html#msg2131598 I didn't even think I could be transgender until Easter this year, and even then, I thought maybe I would just like to actually be a girl, but not really see myself as one.
Sometimes you know for a long time, sometimes only your subconscious really knows. It's kind of surprising how the brain works like that, I think. I was looking at full transition videos on youtube and stuff long before I ever considered and let's just say I had done
nothing up to Easter this year to support myself in a female role. I didn't even really bother studying clothes, makeup, or anything, really!