Puberty blockers are the standard medical treatment and depending on where you are they might be free. You'll need to get it prescribed. Blockers are this kind of thing
https://en.wikipedia.org/wiki/LeuprorelinThey are fully reversable, so that if you stop taking them puberty continues as normal.
I read a paper a while ago that said, as best as I can recall, there's some bone density loss, but its not dangerous and (better than the alternative). Long tem use when you're older is dangeous and can lead to ostepoprosis (bad).
I don't know why the difference between before 18 and after, but I suspect its either because its relatively short term before 18, or that doctors are willing to risk osteoporosis but not the social backlash from giving HRT to minors.
I've often wondered, but I don't know why the heap antiandrogens (like spiro) are not used instead for minors.
There was a post on another forum I read recently by a women who managed to castrate herself. She almost died, but someone called an ambulance and she got to hospital just in time. The guy who did it was arrested and the entire thing made the news.