They say you're born TS. Some will know as a child while others like me will know something is wrong but can't pinpoint it because of the way we are raised. I couldn't figure out myself until my twenties. I couldn't understand the concept of having male anatomy and wanting to be a girl. How could I possibly be anything but a male? It took me until my twenties to realize who/what I was. Then life got in the way and I buried all thoughts of ever getting it corrected. There was no internet or help in those days.
At what age did you know you were TS?
*EDIT*
Simply when did you come to the conclusion that you are (or were for those that have transitioned) TS? The AHA moment, the realization. The inescapable moment of clarity when it all made sense. I don't think we can remember the exact day of course.