There should be no legal age to buy anything... the law has zero business forcing its will on any U.S. citizen doing anything in the pursuit of happiness (that doesn't affect the free will of others).
As I've heard SO many times before when it benefits large corporations... if you remove government oversight and restrictions then the market will take care of itself.
What?
It doesn't work for cigarettes and alcohol? Then it doesn't work for anything else, either.