Is someone permitted to drink alcohol or use weed when they are that young? And those things are less permanent than a full sex change, which cannot be fully reversed. And yet, why does one need to be 21 to drink, yet pre-teens can get sex change operations?
I would prefer to make it 21 for such an operation. Dressing in cross-gender clothing is something that can be reversed in about two minutes without any lasting damage. An operation like this cannot be reversed, and if they decide after all they wish to return to their original gender, they are s*** out of luck.