This is always an interesting question. The answer should be 19 (18, just barely), but a few weeks shy of my 21st I put it all on hold. Maybe only for six months but it didn't work out like that. So then it turned out to be about 29. But then... 26 when I decided it wouldn't stay on hold and I wanted to get a place far away from my home-town - something that caused major problems when I was transitioning originally - and that dragged it on a couple years or so... and the irony is, for the most part I had to suffer being stuck in that town anyway. All that messing around was for NOTHING.

I think better support when I was late teens would have made a MASSIVE difference. I can't imagine. I had alleged support from one person but actually they either were or were not utterly dishonest to me (depending on how you interpret it) - but it knocked my head in regardless. So much that I didn't tell anyone else for another eight years, roughly*. And that person dumped on me too eventually, but, whatever.
(*That isn't quite true. I told someone in London in 1994. I got blind drunk at some event and found myself wondering the streets aimlessly in the early hours [DON'T ASK!!]. This nice gay fella rescued me - took me for a coffee - and grilled me a bit. He kinda figured it out somehow - he eyed me squarely and said, what's the deal, you're not gay, but you sure aren't straight - so I admitted it. I said, well, I was getting a sex-change a while back but I sort-of stopped. But I had no idea who he was so it didn't seem to matter... just a random thing. BTW, he said no way would I be 'stopped' forever and gave me a wink. LOL. So if the guy with greenish hair from London, Sept '94, ever improbably happens to read this - never caught your name - hi! Never told that story in public before.)
Anyhow it was 15 years ago. I can't believe how old I am now. But that's a whole other can or worms for elsewhere, maybe.