"Gender dysphoria is a mental disorder listed by the world recognized..."

And less than a century ago, there were many learned organizations and individuals who insisted that homosexuality was a form of mental illness and those inflicted with this condition were in need of treatment. 

Less than a century from now, we may look back on this issue and say ... (fill in the blank.)

Conclusion: we just don't know enough.  To make it more complicated, we're dealing with human beings and that means many will defy "standardization" or "generalization" or "approach."

So, I think for that reason, the headline's question should be changed from "Who decides" to "When to decide?"  Ans: at age of majority.  Which automatically means, the person him/herself. "And thanks, Mom and Dad, for allowing me to make my own decision."