Many countries have no drinking age; in other countries, the drinking age is 18. The US, along with just six other countries, has a drinking age of 21 with extremely strict consequences for minors.
If an establishment is caught selling alcohol to someone underage, they could lose their liquor license, which could incur huge fines and be extremely damaging for business
. Some travelers note that people in other countries are more likely to let you slip by if you look around the drinking age, especially since wine flows so freely elsewhere, but in the US, they may ask for an ID from someone with a full beard.
Of course, this varies from establishment to establishment, so you should keep your ID handy no matter where you are.