Because kids are impressionable and easily manipulated.

It's why there is an age of consent that no kid can waive.

It's why kids aren't allowed to do lots of things, like vote, drive, drink etc.

There needs to be a compelling reason to make an exception for one politically charged thing that comes with irreversible physical changes.

There also needs to be a compelling reason why that decision cannot be deferred until age of majority.

the child can't consent to medical procedures without a parent, today, and if the parent says no, the decision is deferred until the age of majority.

> he child can't consent to medical procedures without a parent, today, and if the parent says no, the decision is deferred until the age of majority.

What does the parent's opinion have to do with it? Their opinion is equally ignored when it comes to statutory rape, no?

This is not a "this is a private matter between the child and the parent" scenario. Some things are, but a clear majority of people do not feel that this should be one of those things.

Convincing and teaching minore to synthesize hormones which will chemically castrate themselves / make them infertile and cause permanent non reversible damage to their bodies, how to use those hormones, how to hide it from their parents. These are all bad things, and the people who do so should be in prison. If I taught kids how to make weapons secretly and told them to use them, I would go to jail.

It appears you and I have different definitions of “sharing HRT tips”.

Because youths don't have developed critical thinking skills and HRT is an invasive medical treatment!?

Putting aside that MDs trained in medical ethics should be the ones to decide the end-all debate of HRT for those under 18y/o (or maybe 24 by your standard) -- I would imagine "tips" here is mostly about logistics of navigating the US health system and filling in health-effect anecdotes where science has yet to affirm/study (which encompasses more areas of health than you might think). Also, "Invasive" and "non-invasive" are usually reserved for surgical contexts so I'm not sure I would apply that here.

Zero kids are getting HRT without their parents consent, don’t spread transphobic FUD

It's not about the kids of their parents, it's about other parties that are promoting or encouraging HRT to other people's kids.

Because kids are highly impressionable and have limited ability to make sound judgements?