Age restrictions are good for things like alcohol as it is a dangerous substance if not used responsibly. If you drink too much then you destroy your liver. But having it too high is being a bit TOO restrictive. 18 is a good age to have it as thats when most people start their adult life and are able to make their own decisions. 21 is a bit too high as it makes it seem that you are only able to be an adult at that age. Its a bit hard to explain my reasoning to others but I hope this is a little coherent :P
Alcohol is considered one of the most addictive substances available in mass quantities to man. It is also quite poisonous to the human body. Statistics show that 40% of all patients in hospitals are admitted due to complications from alcohol consumption. Alcohol is also one of the few substances that can kill a person from substance withdrawal. Children are this world's future and should be protected from exposure to such things.
However nice it would be to have a more European style view of alcohol, in the United States we have developed a specific culture around alcohol which leads to dangerous actions when finally able to drink. Because of this taboo, there must be age restrictions, though it should be lowered to 18 to be more in line with other things, such as smoking and military work.
The age of 21 used in the United States is way too high, especially because minors become adults for all other purposes at age 18. I think it is more realistic and even safer for kids to learn about drinking legally and in more protected environments at an early age. Sixteen makes sense since it is the common driving age.
Alcohol is a very dangerous drug in that not only does it adversely affect body functions but it also deeply affects our reasoning. Alcohol related traffic accidents kill thousands of people every year, and allowing alcohol usage by even younger people will only increase such fatalities. Alcohol should only be used by fully developed adults who are aware of the consequences of their actions.
Alcohol has become a great problem towards minors. While having the age restriction, children under 21 years of age look at alcohol as the sacred drink, thinking that it will be the best thing they have ever tasted and are tempted to take it at an early age. With this temptation towards under-aged children, when they get caught, they get in serious trouble. If the drinking age is lowered, children will start to not think so seriously about it and they will not be so tempted to drink it.