All ages except minors. So yes, like all other things in society, people who are incompetent to make their own choices, good or bad, are restricted from that until they are competent. So, people pick arbitrary ages along with restricting the insane and retarded and senile.
In short, in a rational situation, someone as crazy as you would be restricted. :^}
So you would allow the government to pick that age? They decide that say, a 25-year-old is competent to do drugs and one who will be 25 next month is not?
You wouldn't allow the individual to prove personal competence, nor would you leave the decision to the parent. You would allow the government to make that decision.
Fine. Just so I understand. (As though anyone could understand such a convoluted, self-serving explanation.)