> perhaps level of education attained, demonstrated ability to take responsibility for themselves, etc.

Yes, that is what I meant. Choosing some arbitrary magic number like 18 and declaring that anyone under that age is too dumb or too irresponsible to decide is about as stupid as declaring that anyone over 65 is incapable of decision making because they might have Alzheimer's.

Ok, but this doesn’t scale well. It takes a nontrivial amount of time to evaluate each person, and it would have to be done repeatedly as they age. You would need an entire government bureaucracy devoted to it. Then people will want an appeals process for when decisions don’t go their way—both parents and children.