Northern Light
Superstar
To be clear, the US doesn't have a 'national' drinking age; that is a State matter. But the federal government does use tax and financial levers through the Federal Uniform Drinking Age Act (1984) to keep the states in line. A state could have a lower age but they like the money. Similarly, the Safe Teen and Novice Driver Uniform Protection Act (2011) does similar with federal transportation funding. Money talks.
For further clarity, the age of consent is also set at the State Level and it varies.
However, a number of federal statues use 18 as a delineating age for whether something secondary to sex itself is an offense or the severity of an offense. (or sex in certain circumstances)
As a post-script......... the minimum age for marriage 15 in Kansas and Hawaii. In Mississippi this only applies to girls, boys have to be 17.