I can see that risk, if other people are taking the decisions for you then it's wrong. But this plays both ways. Making transgender children wait, in effect indefinitely, is also wrong for more or less the same reason.
Also, there's a lot of moving of the bar here. A letter that no doubt represents an opinion shared by many is that even 18 isn't young enough to make the decision to undergo medical treatment for transgender care, and instead suggested that 25 should be the cutoff, in order to give the brain further time to develop. But why stop even there? Why not 30?
The basic flaw behind a lot of this reasoning is that people see being transgender in the first place as a mistake, a problem, a lie, whatever, and are evidently determined to stop people from making this "mistake" if at all possible. I certainly agree that if, on a Tuesday, a young child questions their gender identity, then rushing them onto hormone treatment on Wednesday and surgery on Thursday is wrong. But the converse approach, of trying any and all excuses to kick the can down the road, is equally flawed.
I'm sorry for what happened to this child. But the answer isn't to ban gender-affirming care, for teenagers or anybody else.