Ideally they'll start hormones or go off blockers by about 16, but you can't comprehend that because you already decided that 18 had to be the start point. You don't want trans kids lucky enough to discover their identity early to transition, probably not thinking through what the consequences are. You hope that they'll not medically transition, but in all likelihood, they'll just end up suicidally depressed for no good reason.

You're correct in assuming that gender isn't a choice, but incorrect in how you apply that idea. Attempts have been made to force cis boys that got maimed by botched circumcisions to be girls. They did not end well. Attempts have been made to find alternatives to gender affirmation, but they never work. Nobody can be converted. Nobody knows how.

Stop being a knob and accept the failures of naive doctors trying to snuff us from existence. You affirm us, or you torture us. There is no other option.