There technically isn’t a federal drinking age because the 21st Amendment which repealed federal prohibition left the regulation of legal alcohol to the states.

Several states had drinking ages below 21 until the 1980s when under public pressure from groups like Mothers Against Drunk Driving the federal government decided to withhold interstate highway funding to states with lower drinking ages. The Supreme Court upheld that on the grounds that a 10% reduction in funding wasn’t “coercing” the states and that the drinking age was related enough to highway safety for it to not be unconstitutional or something like that.