It's one thing to say "Look, I know that you want this surgery, but it is a huge change and you need to take some time to think about it. Wait until you're 18, just another year or two, and if you still want it, we'll support you and help you transition."
It is another thing to say "You're f**k and you're not getting it. No matter what you do we won't support that decision."
People seem to forget that all human beings are basically retarded until at least 25, and a substantial portion never stop being retarded at all. I highly, highly doubt that any 16 or 17 year old has the capacity to make such a drastic decision. The parents were right to say no at this point, assuming that was their logic.