I think 18. You gain full responsability as an adult and its still young enough to live a happy life with whatever you choose to do. Before 18 ..I would be scared people would not be sure enough on such a thing.
Ideally, i would agree with that. As in age wise. However, once you've gone through puberty, irreversable damage (which is exactly what it is if youre in the wrong body) has already been done to your body, and any hormones taken after this point will have a far lesser impact than if you took them before puberty. And seeing as now there is such a thing as 'puberty blockers', there is no reason to make a 13/14/15 year old go through an experience they vehemently don't wish to endure. Then afterwards, if it is just a "phase", the puberty blockers can easily stop being taken, allowing you to go through a normal puberty. No damage done there. With these puberty blockers, i see NO reason why any young person who feels they are in the wrong body should be subject to the torment and and irreversable effects of puberty. I would definitely have taken hormones before puberty if i had that option, but failing that, the puberty blockers would have been very welcome as well. I only say this, as the effects of puberty makes it much harder to "appear" female, than if you didnt go through it. I am talking from bitter experience, so i should know.