mostly 18, sometimes 16. But in the United States, it's 21. Main reason why American college students leave the country for spring break. Also proven to be a trap for international students coming to study.
Scenario 1 (local American student)
Bar tender: Can I see an ID?
Student: (hands over fake ID)
Bar tender: Looking at your ID it does not appear to be real, I am sorry but you have to leave.
Student: WTF!?! I am seriously sick and tired of waiting until 21. I am going to Canada for spring break where the drinking age is 18!! Maybe party in Montreal! Plus they are generous to accept my American drivers license there.
Scenario 2 (International student studying in the US)
Bar tender: Can I see your ID?
Student: (hands over a European passport)
Bar tender: I'm sorry but you gotta be 21 to drink
Student: WTF!? I thought you have to be 18
Bar tender: Nope, it's 21, and it will always be
Student: Then I was tricked, in my country I can drink at age 18. The US has the stupidest drinking age ever and they seem to hate college students like us...