I was what could be called a "true trans child". That's even the reason why I ended up transitioning as an adult: because I could remember "feeling that way" for more than 30 years, so there was no reason to assume it was ever going to change
This is where a fundamental difference in belief comes into play. To someone who believes in gender identity, this is plausible. To someone who doesn't (like me), there will always be an underlying explanation for this kind of distress.... and it will always be possible for it to be addressed in a different way from transition. However, it's never that simple really because belief is incredibly powerful.
As an analogy, I was talking to a (now former) work colleague in Eastern Europe about this type of thing and he shared an utterly heartbreaking story about the experience of a friend of his. The friend was a doctor and had to respect the wishes of his patient's parents (the patient was a young child) who were orthodox Christian and had chosen to refuse his medical treatment on religious grounds. The doctor had to listen to his patient saying he didn't want to die 😓 The child died not long after 😓
To my own ethical standards, any medical treatment (or withholding of treatment) where the consequences are loss of health/life as a result of a belief that someone holds are wrong. However, if the parents had been witholding their own treatment and the consequence was their own death, I would have reluctantly accepted that they chose this of their own free will. I would still find it sad (that their belief led to this) but in a different way.
If someone believes themselves to be "true trans", remains happy throughout their life about their transition (no matter what the health consequences), doesn't require anyone else to "affirm their gender" and was over 25 at the point when they did anything permanent, I would feel a similar reluctant acceptance (but without the sadness if they are still alive and it doesn't shorten their life) IYSWIM.
I accept that my own standards of belief (or lack of, in both cases) don't hold any sway over their fervent belief. None of this feels easy or simple ethically, but that's the point at which I hit my "live and let live" threshold.
They should be made to understand that they will have to wait until they are adults before taking any lasting decision.
Personally, I would rather no medical options were available at any age. However, that's going back to my own (lack of) belief standards, so my reluctant minimum would be 25. I would only accept this if the person had received truly neutral support to unpick why they felt the way they did about themselves. To be truly neutral this would have to be without any active affirmation of any kind (e.g. complete pronoun avoidance, so that the person feels neither upset at sex-based pronouns nor "colluded with" via preferred pronouns) and would be about exploring the physical dysphoria and whether it may have a trauma, sensory or other origin.
My minimum age has moved upwards, the more I've learned about the harms associated with "gender affirming care". Previously it was 18, now it's 25 with a preference for a complete ban.
From my personal experience, if I had to declare one environment to be an echo chamber, it would be the trans community, certainly not this board.
I can well imagine.