Why is the drinking age 21? Where did the 21 come from? It seems like an arbitrary age.
In 1984, Ronald Reagan signed the National Minimum Drinking Age into law. This law told the states that if they did not change the drinking age to 21 they would lose 10% of their federal highway funding. Eventually all the states fell into line, and this led to a reduction in the number of drunken-driving deaths, so it seems the law had a good effect. I know you may be wondering why a person who found this sneaky way of forcing states into changing their laws to align with federal policies - even though the 21st amendment defers the power to regulate alcohol to the states - is considered to be the hero of limiting federal control over the states, but I don't want to argue about it.
The question here is how did 21 come to be seen as the age of majority in this case. Why not 20 or 22? The absolute origin of 21 as a magic age is lost to antiquity, but it is clear that it came to our country from English Common Law, which was once the law of land here in the states, where our legal heritage originates with the original 13 colonies, which were all bound by British law.
You may ask, What is English Common Law? A quick answer two this is that the British do not have a constitution like we have: a document that was written down and you can go and read. Their law consists of statutes that have been passed by kings and parliaments throughout history (known as "statute law"), and various legal judgments and conventions (known as "common law").
English common law gave people increasing responsibility as they aged. Common law took into account the fact that girls mature more quickly than boys, so they do not always get rights at the same age. Men began to gain rights at the age of 14, while girls would begin at 12. For example, a man could sign a contract at 14, but could choose not to ratify it at 21, even though an adult who entered a contract with a minor was bound to follow it. It raises the question of why anybody would enter a contract with a child since the child could so easily abrogate it, but that is the law. A man could marry without parental consent at 21. An interesting rundown of various rights by age can be found here; I can't vouch for its accuracy, but it is pretty interesting.
Outside of England and its colonies, the age of 21 doesn't have the magic power as it has here. There are a handful of countries that have a minimum drinking age of 21. All of these countries are either former British colonies, Islamic countries who have additional restrictions concerning who can purchase alcohol, Pacific islands that have a history of close ties with the United States. Taking a world-wide perspective, a drinking age of 21 is very rare. 18 is the most common, although some places have 19 or 20. Because of our success in reducing accidents by raising the drinking the age, there have been attempts in places like New Zealand, Australia, and the Philippines to increase the drinking age, but we are still relative outliers.
An interesting sidenote is that the original Selective Service Act of 1917 required men to register at 21. The age was lowered to 18 in 1918. Since some people object to men being required to register at 18 even though they are not allowed to drink until 21, maybe Selective Service should be changed back.