Very few countries have a legal drinking age of 21, with the USA being one of them. It hasn't always been that way, the legal age to drink in America use to be 18, but that was overturned in 1984. Changing the minimum drinking age from 18 to 21 was a travesty of justice for several reasons which I expand upon below.
In 1971 the 26th Amendment to the Constitution was pass, which enabled the voting age to drop from 21 to 18 throughout our nation. This was an important step in enfranchising those that did not have a vote in a time in which they were subject to a draft at the age of 18. This also confirmed that for the vast majority of states, the legal age in which you essentially moved from being treated as juvenile to an adult was now at 18. Consequently, we can conclude that at age 18, decisions that you made, were no longer mitigated by being a juvenile, yet this appears directly contradicted by the drinking age moving from 18 to 21 in 1984.
Either you are an adult at age 18 or you are not. As an adult, you should be able to engage in all activities that other adults have the right to participate in. By taking adults of the ages 18-20, and arbitrarily deciding that they cannot consume alcohol base on nothing more than capricious whim, you have created a 2nd class of citizenry in which these particular citizens are having their 14th Amendment Rights violated. The 14th Amendment states:
"No State shall make or enforce any law which shall abridge the privileges…. …nor deny to any person within its jurisdiction the equal protection of the laws"
Additionally, there is a misperception which you often see in laws, that by virtue of passing a particular law, that you will modify or change behavior so as to not violate that new law. In fact, what happens when you pass a bad and unjust law is that you take what use to be normal law-abiding citizens and make them law-breaking citizens by virtue of a bogus law. This gives the police and courts more power over citizens since they can apply the law at virtually any time against citizens as a form of arbitrary punitive punishment.
It is not for me and you, to decide whether a young adult of ages 18-20, should or should not consume alcoholic beverages. If alcohol consumption is pure evil, than why is it tolerated for those that are 21 and above. There are several countries that do not permit alcohol consumption whatsoever; why not try to pass a Constitutional Amendment to that effect and thereby make the law equally fair.
If the United States wants to keep the age to legally consume alcohol at 21, then why not change the law to define all those that are under the age of 21 as juveniles, and subsequently change all laws to reflect that fact.
If not, it seems clear to me, that if you are old enough to be drafted or to serve your country at age 18, you should also be old enough to legally take a drink.