In short, we ended up with a national minimum age of 21 because of the National Minimum Drinking Age Act of 1984. This law basically told states that they had to enact a minimum drinking age of 21 or lose up to ten percent of their federal highway funding. Since that's serious coin, the states jumped into line fairly quickly. Interestingly, this law doesn't prohibit drinking per se; it merely cajoles states to outlaw purchase and public possession by people under 21. Exceptions include possession (and presumably drinking) for religious practices, while in the company of parents, spouses, or guardians who are over 21, medical uses, and during the course of legal employment.
That answers the legal question of why the drinking age is 21, but what was the underlying logic of the original policy?
Did lawmakers just pick 21 out of a hat because they wanted college seniors to learn the nuances of bar culture before graduation? Not quite. The concept that a person becomes a full adult at age 21 dates back centuries in English common law; 21 was the age at which a person could, among other things, vote and become a knight. Since a person was an official adult at age 21, it seemed to make sense that they could drink then, too.
Who was responsible for lowering the drinking age to 18 for part of the 20th century, though?
Believe it or not, Franklin Roosevelt helped prompt the change in a rather circuitous fashion. FDR approved lowering the minimum age for the military draft from 21 to 18 during World War II. When the Vietnam-era draft rolled around, though, people were understandably a bit peeved that 18-year-old men were mature enough to fight, but not old enough to vote. Thus, in 1971 the states ratified the Twenty-sixth Amendment, which lowered the voting age to 18. Legislators started applying the same logic to drinking. The drinking age, which the 21st Amendment made the responsibility of individual states, started dropping around the country.
Critics of the change decried rises in alcohol-related traffic fatalities among 18-20 year-old drivers in areas where the drinking age had been lowered. Indeed, one result of leaving states in charge of their own age was the creation of "blood borders" between states that allowed 18-year-olds to drink and those that didn't. Teenagers from the more restrictive state would drive into the one where they could buy booze, drink, and then drive home, which created a perfect storm for traffic fatalities. Even if teens weren't any more predisposed than older adults to drive after they'd been drinking, all of this state-hopping meant that those who did drive drunk had to drive greater distances to get home than their older brethren, who could just slip down the block for a beer or six. More miles logged in a car meant more opportunities for a drunken accident.
Who led the back-to-21 movement?
Organizations like Mothers Against Drunk Driving began agitating for a uniform national drinking age of 21 to help eliminate these blood borders and keep alcohol out of the hands of supposedly less-mature 18-year-olds. As a result, President Reagan signed the aforementioned National Minimum Drinking Age Act of 1984. MADD's "Why 21?" website touts a National Traffic Highway Administration finding that the raised drinking age policy saves around 900 lives a year. Traffic reports show a 62% decrease in alcohol fatalities among teen drivers since 1982. Raw numbers show that drunk driving fatalities have definitely dropped since the early 1980s; despite an 88% increase in the number of miles driven, 2007 saw over 8,000 fewer total alcohol-related traffic fatalities than 1982.
Teasing out the underlying cause of this reduction in total fatalities is no mean feat, though. Non-alcohol traffic fatalities have also declined relative to the number of miles driven over the same time period, which could be attributed to any number of causes, including increased seat belt usage, the widespread use of airbags, and other safety improvements to cars and roads. Moreover, drinking and driving for the whole population might be down as the result of increased education on its consequences, harsher penalties, improved enforcement, or increased stigmatization of drunk driving.
College presidents who supported the Amethyst Initiative—a movement launched in 2008 to reconsider the national drinking age of 21—admit that drunk driving is a serious problem, but they point out that it's not the only potential pitfall for young drinkers. They contend that by lowering the drinking age, colleges would be able to bring booze out into the open and educate students on responsible consumption. Such education might help curb alcohol poisoning, drunken injuries, drinking-fueled violence, and alcoholism on campuses.
Interesting bit of trivia: the group takes its name from the character Amethyst in Greek mythology. She ran afoul of a drunken Dionysus, who had her turned into white stone. When the god discovered what he'd done, he poured wine on the stone, turning it into the purple rock we know as amethyst. Ancient Greeks wore the mineral as a form of protection from drunkenness.
So is there any place in the United States an 18-year-old can escape the uniform 21-year-old drinking age?
For a while, Louisiana was a safe haven for thirsty teens. To comply with the letter of the national drinking age law, the state passed a law that made it illegal to buy alcohol if you were under 21. However, the law had a pretty large loophole built in: it wasn't illegal to sell alcohol to people under 21, a trick that severely hampered the enforcement of the drinking age. The state closed this exception in 1995, though. For the truly creative, the National Institutes of Health note that since Indian reservations are domestic sovereigns they don't fall under the existing federal drinking laws. Don't start researching your own tribal history just yet, though; according to the NIH over 200 tribes have passed their own laws against underage drinking.