I see your point, and it does have some validity. To be honest, I don't think there is a right answer.
That said, there are some reasons for allowing teens to undergo hormone treatment, especially doing so before the major onset of puberty. Everyone knows puberty makes changes to the body. Allowing (say) an FtM to start hormones before he grows D-cup breasts, or allowing an MtF to start hormones before her voice drops to baritone would be very helpful. (Reduces the amount of changes that have to be reversed surgically.) Also, the bone ends fuse in the early 20s. If hormones are started after the bones fuse, the skeletal system will not change. FtMs will have women's hips, and MtFs will have male facial structure. If hormones are given while bones can still change, again, it is less that has to be reversed, if they can be reversed. (MtFs can have painful facial reconstruction. FtMs cannot reduce their hips.) FtM's who are short (or even average) for women will be VERY short for a man.