I largely agree with you that it is an important and significant distinction, but indulge a little thought experiment in which I try to show that even age-based

I largely agree with you that it is an important and significant distinction, but indulge a little thought experiment in which I try to show that even age-based thresholds can result in undesirable outcomes. What if the age of agency were age 100? Only about 1 in 10000 Americans makes it to that age. So in this world, only the very-long-lived might have access to lucrative investment opportunities. Wealth would accumulate in families with a genetic predisposition for longevity (which is itself correlated with wealth! a positive feedback loop). This seems undesirable to me. One might say "well sure, but 99.9% of people live to 18, so that is a much more reasonable number" (and I obviously agree). But then why not 16? More people live to 16 than 18, tautologically. Why not 10? Because there is threshold of "mature enough" that we, as a society, decide on, and this threshold is completely arbitrary.