Thank you for asking such a fascinating question.
Many cases of gender dysphoria start in childhood as most of us will have a very good idea of our own gender even at a very young age like a few years or so. At age 3, we would begin to develop our own ideas about gender.
In clinical practice, there are cases where they only seek help when they are adults. When I probe further and ask them about their discomfort with their gender, the typical history is that they feel trapped in the wrong body and this discomfort starts when they were only a few years old. Psychiatrists will also ask them whether as a kid they enjoy playing with friends of the same or opposite gender and what kind of toys they enjoy playing the most with. A history of cross dressing is also relevant. Oftentimes these persons try hard to deal with the problem on their own and suppress their inner desires as they do not want to cause embarrassment to their family members and friends. It is only much later when they give up trying, or when they are too tired to keep suppressing their preferences and desires, they then decide to seek medical attention to allow their true selves to be manifested. They are tired of being bombarded with gender stereotypes that conform us to culturally expected norms, and decide to seek help from the healthcare professions. Sad to say, by then, some cases were already married and have their own children. The problem then becomes more complicated as any treatment recommendation will have a direct impact on their spouse and children. Hence, my recommendation is that persons with gender issues need not suffer in silence; they should seek treatment early.
I hope this answers your question. Cheers! Take good care.