The National Minimum Drinking Age Act of 1984 required all states to raise their minimum purchase and public possession of alcohol age to 21. States that did not comply faced a reduction in highway funds under the Federal Highway Aid Act.
I guess DC doesn't count as "all states" but it certainly is the case:
I didn't go to law school, so I can't quite make heads-nor-tails of the 'history' section.
/jordan