The minimum legal drinking age in U.S. states is 21 instead of 18 because studies have shown it leads to fewer traffic fatalities. Most people drive on public roads, and want them to be safer. They realize that too many young adults are incapable of being responsible with alcohol, and they want to do something to prevent drunk driving. So these laws get voted in to protect people's safety.
These laws are particularly unfair to the minority of people, aged 18-20, who don't regularly drive on roads. College students who live on-campus and young adults who live in inner cities often don't own cars. It's absurd to outlaw alcohol for these adults. Most reasonable people can see this—however, they are okay with keeping the law in place because it is acceptable to them to oppress the small minority that doesn't drive as long as it leads to safer roads. It seems unfair to discriminate against a minority lifestyle, regardless of how irresponsible the majority is. There is nothing wrong with walking, biking, or taking public transportation—the only relevant difference is that most Americans do not travel this way. I don't see how laws that make assumptions about people's lifestyle can be compatible with a free society. Why should undergraduates with drinks in their hands have to duck in the bushes every time they see a cop? The minimum drinking age of 21 clearly discriminates against a minority lifestyle.