I knew it was POSSIBLE at a young age. My father was a newspaperman, and had a habit of keeping newspapers from things like the moon landing and such. While nosing through these in our basement one day, I discovered that he had kept an ancient article about Christine Jorgensen. I have always wondered why he did!
Also, Renee Richards and all that tennis stuff was news when i was a kid. BUT, although i realized it was possible to transition, it didn't seem like something one could actually do one's self. it seemed to me to be treated like a circus sideshow, something for the Odd News page.
I didn't think transition was genuinely possible, on a personal level, until I was about 35 or 40. By then I had a life in place, a family. It took me until seven years ago to act on it.