You almost had me until you mentioned MRIs. The human brain is constantly changing from what I understand, from birth till death, constantly shaped by an insane number of variables encountered in everyday life. From your diet, sleep habits, hobbies, sunlight, you friggin name it. So I don't follow that.
However, if you want to keep it as simple as two genders, I'm down. You want to be MF/FM, fine. You can be one of the two biological genders, your choice. Declare it, it becomes legal when you're an adult, no taksies backsies.
I sincerely don't care, just fucking pick one or the other and stick with it.
you can certainly have it be when someone is an adult and the brain stabilizes as a cutoff for when you can do this.