But if you insist:
The entire sex change is a process that takes years. First the individual must realise that they are not with the right anatomy. This typically happens very early - usually before the age of two or three. The coming out process might be easy or it might be an agonising long time. There is benefits to do it early and there is benefits waiting until the age of 18. There is also costs. If one comes out early with parents not able to grasp what is being asked, the child will be up for a wide variety of abuses. If the parents accept, then that is a boon. If the parents are unable to grasp and you wait, you can avoid the worst of the abuse. However, one has to deal with living in the uncomfortable position for years and that might lead to abuse of self - self-destruction, suicide, etc.
If the hormones are given before the age of puberty, you can delay puberty until one has the resources to undergo the change. The benefit of this is that your bones don't change during puberty and once transitioned, you will have a more convincing body structure. Once undergoing the hormone therapies, you are considered to be in the transition.
I remember reading one case where a child had, with consent, undergone the surgery under the age of 10. He was happy with the situation, from the reports I've read. He had started taking the hormones at age four or five. This is very rare, however, and did not take place in the US but in Germany. So yeah. Just one successful case at such an early age.
