We usually don’t let minors get married (without parental permission); we don’t let them enter into contracts, and do other such things because they’re MINORS, and have no sense – probably really won’t have any until they’re around 30 years old, if then.
But apparently minors (some as young as 3 years old) can make informed decisions about their gender, and some people are ok about letting them start medical/psychological activities that support that.
If you’re as old as me, you remember ‘old enough to fight, old enough to vote’, a reaction to the Vietnam war – you could get drafted, but you couldn’t vote. Dropping the voting age to the draft age made sense to me! So maybe we need to take another look at when people are old enough to make important decisions about their life, and stick to it. Let’s let 12-year-olds drive cars, take out home loans, get married, vote, etc. Or not. But just fix on a number where you’re supposed to be competent to make life-changing decisions.