The issue is that the person could have been given the wrong sex from birth (it happens)...
Invasive procedures are not taken lightly and extremely rare under the age of 18... If it happens that young then there is a lot of medical professionals and psychologists involved on what would be considered a issue at birth...
As for trans playing sports, there is a hormone limit that all has to pass... I will point out one thing, Russia put in a complaint in the last world championships about two boxers and that they were trans and there hormones were too high. They were striped of there medals without even a test, just on the words of the Russians. When born both women were identified as female and that was never questioned.
What was really damaging is that the Russians brought it up again it the latest Olympics... That is disgraceful...