As long as my chest was reasonably concealed, I was read as male a lot pre-T, but that phenomenon became less and less frequent as time went by, mainly because of weight gain, I think.
On T, I started being consistently read as male from the 3.5-month mark on, but I was already middle-aged. So I was forty-six, but people thought I was in my twenties. I'm past fifty now. People still think I'm much younger than I am, but I figure that my age will catch up with me eventually. Still, if I colored the gray in my hair, I have no doubt that people would once again read me as being maybe thirty-five.
The age vs. appearance disparity is just a fact of life for most of us. I think it sucks most for the younger guys; I don't mind it myself except when people make the erroneous assumption that I haven't had many life experiences. So I'm keeping the gray!