In the US at least, the 21 yo drinking age is mostly a wholly pragmatic law [EDIT: laws. It's state by state but there was a big federal stick attached to highway funding.] to reduce teenage drunk driving (which affects more than the individual). Tobacco sales is mostly 18 in the US (although a few states are 21). Casinos also vary although the drinking age restricts <21 yo in a lot of casinos.
It's hard to make an intellectual case for disparities between things like voting age, military service (which is why the US voting age was reduced to 18 in the first place), drinking/gambling/smoking, age of consent, etc. but pragmatism still sometimes wins out.
It's hard to make an intellectual case for disparities between things like voting age, military service (which is why the US voting age was reduced to 18 in the first place), drinking/gambling/smoking, age of consent, etc. but pragmatism still sometimes wins out.