The MLDA and its real effect

The legal age when young Americans can start drinking alcohol is 21 in the United States now. However, I don't see that the MLDA (Minimum Legal Drinking Age) has real influence on young people because most young Americans have already tried alcohol drinks by the age of 21. Moreover, many of them drink alcohol regularly. Adults and police pretend that everything is under control and young people follow the law. But they know that it is a normal practice when many high school and college students drink at parties. It is illegal theoretically and legal in practice. What's the point of having this law then? I think it should be changed. The government should lower the MLDA to 18 years. It would make legal what is already half-legal. Another reason for changing the law is that college students are mature enough to make decisions about alcohol drinking. They have all rights of adults but can not drink a sip of champagne on friend's wedding or a bottle of beer when watching a football match. Do you think it is fair?