The legal drinking age is 21 throughout the United States (and 19 within Canada except Alberta, Manitoba, and Quebec where it is 18). While some countries like Great Britain are more flexible with alcoholic consumption by minors, restrictions on under age drinking are based on sound psychology and body physics. Young people need to understand the risks associated with excessive alcoholism, particularly on their young and developing bodies.
As children get older, they wish to participate in life. They see their parents or young adults out, having fun on the town, and wish to be apart of that same lifestyle. To go to parties and drink excessively is seen as a sign of independence and social status, and teenagers in particular crave emblems of an unrestricted life. They want to seem like adults, and perhaps even get ahead in the competition for friends and prestige.
While alcohol is pleasurable in itself and seems like an unavoidable part of the party lifestyle, under age drinking is still illegal in many countries, and there are risks that go along with it. It is possible to make embarrassing or even very dangerous choices while drunk, and the bodies of teens are smaller and therefore more vulnerable than adult bodies. While youngsters try to ignore the fact, they often do lack good judgment, and will often pressure each other into consuming very unhealthy amounts of alcohol.
Like any drug, the effect of a drink depends on the alcohol content and the body weight of the drinker. A person who weighs more has more body fluids, and alcohol mixes with the entire blood stream. A heavy person will feel the effect of a drink less, because the drug is more diluted in his or her bloodstream. On the other hand, the same amount will take as long to filter, because the liver can only process so many toxins at once.
Under age drinking is a problem because teenagers frequently weight much less than adults, and so are much more sensitive to alcohol. An overdose can cause a person to pass out, suffer brain damage, and even die. Imagine the effect of binge drinking on someone weighing less than 100 pounds. Dangerous choices mixed with a low body weight is bad situation.
One other concern is that the body chemistry of teenagers is different. Underage drinkers sometimes forget that their bodies and brains are still developing, and tinkering with body chemistry while it is undergoing complex changes can sometimes have unexpected consequences. While the evidence is sketchy, consuming alcohol as a teen can affect brain development.
Older folks in the community might see teenagers as rowdy hooligans that need to be controlled, and alcohol in their hands is just one more source of trouble. While there is a definite link between drinking and misbehavior, drunken adults are guilty of the same problem. Children should not be over-wary of being "controlled." Under age drinking is a bad choice and discouraging it is good for long term health.