Following is a brief rundown of legal drinking ages around the world (see a more complete list of legal drinking ages worldwide at International Center for Alcohol Policy). Remember that if you're old enough to fight wars, drive cars and vote, you're old enough to buy alcohol in most countries -- there's a reason you'll be treated like an adult with adult smarts and privileges elsewhere in the world. The joys of a pint of ale in London or a glass of vino in Italy are well worth exercising the self control 21-and-unders are assumed to possess and expected to demonstrate elsewhere on the planet.
Exceptions to legal drinking ages exist in many places (including the US) -- you can drink alcohol when you're with your parents in some countries, for instance. And the island of Puerto Rico is a US territory (meaning no passport is needed for travel to that piece of the Caribbean), but the legal drinking age is 18.
Drinking Ages Worldwide