A fair solution to this is: you get to decide what gender you are at 18, and we’ll recognize it after surgery to match the gender.
If a child cannot get a tan without parental permission until 18 and no tattoos because it is life altering, you don’t get to alter your gender or identity or biology until 18. Then, as an adult, we can’t keep you from screwing up, but when you alter the anatomy to fit the mind, you can be that gender. But a man in a dress is still a man, and a woman in a suit is still a woman.
Then we avoid the “my five year old son wants to be my daughter” and boys capable of impregnating girls sharing quarters with them.
No matter how much you mutilate a boy's body, he will always be a boy.
No matter how much you mutilate a girl's body, she will always be a girl
Humans have only 2 sexes, male and female. There is no such thing as "gender" when talking about people.
Your post following this one has it right. Transgenderism is a mental disease.