A good friend of mine has a son who was born his daughter. I first met the kid when he was 11. He was the obviously-bright kind of kid you'd expect from two brilliant engineer parents who strike me as having the perfect balance of caring for their kids but not being all up in their shit.
When she hit puberty, she had a rough few years before deciding to come out as a man. I can't imagine the guts it took to do that in a rural TX town, but he eventually transitioned and is just wrapping up undergrad in an advanced field with several institutions actively pursuing him for post-grad. He's just a really kind, well-adjusted, extremely ambitious, super-smart 22 year old who's going to make some big waves in a really positive way.
I'd hate to think what would have been missed if he'd not been allowed, or not had the guts to make the call to live life as his true self. No doubt there's been countless unnecessary loss and suffering over the years from people not feeling like they had the option to transition, or not being accepted after they did.
That said, as with most people on here I think any irreversible changes on minors are generally a bad idea. There are going to be exceptions, but they should be rare. For every example of somebody who really needed to transition (and I really don't want to under-count that group!), how many kids are there who are struggling to find their identity and might be willing to dive into something just to set themselves apart?
As a horrendously imperfect comparison, you don't see many 40 year old goths and on that note, teenagers sometimes fuck up when trying to figure out their identity.