Balog, I think you have really identified the issue. There's no question that some limits should be in place, but how to draw them is really tough.
HTG, Parents should not have total control of this - if you had a couple of parents who really wanted a girl but had a boy, so they raised him as a girl and then pushed for a sex change at 14, even if the kid were willing...would that be permissible? That's a pretty decent case for state intervention, as is this one.
I think that instead of rules there should be guidelines that whatever authority is involved has to consider, like:
-the likely impact of the illness (disfigurement, death, etc)
-the certainty that the bad effects will occur
-the certainty of improvement with medical treatment
-the certainty of improvement with the parents' method
It would also be necessary to inquire into the genuineness and nature of the parents' and child's motivations.
This is one where you need to rely on a good decision maker - tough to do, really.