It seems rather odd to me that an 18 year old in the US is allowed to enter the military, be drafted, smoke cigarettes, take on loads of college debt, and buy a house (despite the unlikelihood of that), yet the legal drinking age is set at 21, even though human brain development ends at around 25. What are the reasons for this difference?
The age of majority — in most US states, and for most activities that were considered 'adult' — was set at 21 up until the 1970s. I'm not sure there was a specific reason for that aside from convenience and folk wisdom. It was lowered to 18 largely because of the heavy use of conscription during the Vietnam war; there was public dissatisfaction about sending young men to fight in wars when they could not vote for the people sending them there. The drinking age was raised to 21 once again in the 1980s, ostensibly because of a large increase in drunk-driving accidents traceable to young adults.
There's a lot of other politics mixed in with this: religious concerns about immorality and predatory sexual behavior, conservative worries about the rise of 'psychedelic' drugs in the anti-war movement, lobbying by insurance companies trying to increase their profit margins... But these are the basics.