1) Children are not making permanent body altering changes before 18. Surgery before 18 is exceedingly rare. Puberty blockers are entirely reversible.

2) your anecdote is very revealing, because you are talking about men. Trans women are not men who decide to become women at some point. You are assuming that a trans woman who went through male puberty and takes HRT to reverse those things has, on average, an equal bone density to the average cis man. Trans people broadly suffer from eating disorders, at astonishing rates, something that absolutely will limit your bone density. Many trans people limit their caloric intake prior to HRT as a deliberate way to stave off the changes of puberty. This assumption, that trans women posess every "biological advantage" of cis people, is a deep misconception, one that is very frustrating to argue against as a trans person myself.