The National Minimum Drinking Age Act was instituted in 1984 to prevent the purchase and consumption of alcoholic beverages by legal adults. Many states, including my current state of New York, had legal drinking ages below the age of 21, but at or above the age of 18. The Act was passed to prevent people who were below the old voting age (21 until 1971) from purchasing and consuming such beverages.
As anyone who has read to this point likely realizes, the voting age has remained at 18 for 42 years. However, the drinking age has remained stagnant, because of the convoluted methods used to ensure its longevity.
This act is the sole federal legal document that stands in the way of states making more rational decisions on alcohol within their borders.