I work in a medical school/teaching hospital and just saw a lecture on this topic. It dealt with reconstructive surgery for people who were born intersex and assigned what turned out to be the wrong gender. Interestingly, it listed the age that the subjects first began to experience gender dysphoria, and in every case it was around seven or eight years old. (I would have guessed adolescence.) All of them are doing well now, but they had to deal with a lot of problems because someone went, "Oh, this infant has ambiguous sexual characteristics, better just randomly pick one."