I'd say 16, as most states allow 16 year olds to work and drive automobiles as is. The extension of childhood by the state is abhorrent and should be contested at every opportunity. Instead of raising minimum purchase ages, we should be lowering them across the board and broadly rejecting this idea that the state should have the right to tell you what to do with your body once you're old enough to earn your own money and (theoretically) make it without recourse to parental aid.
The drinking age and the age for the use of currently illicit drugs (all of which should be legalized) should likewise be 16.
Well articulated and I agree 100%.