Several years ago during the battle for transgender rights in Maryland, I was fortunate to have been there during the Senate committee hearings when a woman from PFLAG gave her testimony about her daughter and the problems they had with her from about age 3-4 I believe. They didn't know anything about GD then, nor did any of the many learned men of letters that "treated" their daughter. The right person finally came along and now all is happy in the world and she is now a major advocate for the TG community in general and trans youth in particular.
Many of us have since the age of 4-5 knew something was wrong. About this same age is when a child first gets a sense of self. So it all sort of makes sense that even at that young of an age you can be identified as trans. For others, it may not be clear, or discovered, or acknowledged, until much later in life.