Even though the legal drinking age is not determined by the federal government, there is one, uniform drinking age across the country: 21. This has not always been the drinking age and there were times when the drinking age changed by state. So, what happened? The federal government tied a significant enough portion of a state's highway funds to the state adopting the unified (and often higher) legal drinking age of 21. The drinking age has not changed in the past 30 years.
The thought was that if all the states had a legal drinking age of 21 that there would be fewer drunk driving accidents. While a uniform drinking age may go far to reduce the number of young people driving across state lines to drink or purchase alcohol, there are some people within the motor vehicle safety field who believe a lowered drinking age could actually help drunk driving crashes.