Which would be perfectly sound reasoning if it applied to voting, military service, cigarettes, lottery tickets, etc. If alcohol use is too risky because of an underdeveloped prefrontal cortex, then surely voting is too?

Well for one thing alcohol's effect is to further impair the prefrontal cortex.

But if you raised the voting age to 21 you'd have to raise the draft age, too, because it would be barbaric to send people off to die without even a nominal say in the decision to go to war.

Taboo "barbaric".