I think that's actually a bit of a hard question. Because my reason for not transitioning earlier were twofold:
1. Worried about acceptance (obviously)
2. I had NO IDEA that HRT changed so much.
I had no idea that your face and body could actually feminize to such a degree without surgery. I had no idea that I really could have soft skin and feminine hips and a feminine face and all of that. So part of it also was ignorance, thinking that not even officially becoming transgendered could truly fix the feelings of gender dysphoria that I had, and that I would just end up looking like some he-she-it thing. It wasn't until the very end of last year that I finally learned about this (again, thanks SO much for the "before and after" thread,") and almost as soon as I discovered that it really was possible to become almost completely female, I almost IMMEDIATELY decided that I was going to do it. And if I had known about this as a teenager, I would DEFINITELY have done it then, regardless of what anyone thought.
But on the other hand, part of the reason why I never learned about this was because I was afraid of the societal judgment that comes with it, and as such I never mentioned a word about my transsexual feelings to my therapist or to anyone else that I knew. So had I not felt that societal pressure, I probably would have learned about this much sooner, and therefore probably would have transitioned at least as early as age 14, because that was when I really started feeling like I was going through the wrong puberty, and I officially internalized that what I was feeling was transsexualism.
So it's a catch-22. On the one hand, finally overcoming societal pressures had NOTHING to do with my decision to finally transition. So I'm willing to suspect that if I had known at 14 what I know now about the effects of HRT, I really would not have cared whatsoever about judgmental glances, and would have transitioned without much of a second thought. But on the other hand, it was societal pressures that kept me from discovering this in the first place, so it indirectly did lead to it.
Anyway, kind of an odd experience, but that's what I've been thinking. (I REALLY do wish I had known about this sooner. Now I've got an extra 12 years of testosterone-poisoning to overcome during my transition because I didn't know as a teenager.)