The National Minimum Dirnking Age Act forces states to pass laprohibit persons under 21 from purchasing and publicly possessing alcoholic beverages. If states do not comply, they lose 8% of their federal highway funding (formerly 10%).
The tenth amendment to the US Constitution states that "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people." The Constitution never gives the federal government the power to control the drinking age -- the only reason this law is at all legal is because of the way in which it utilizes a loophole to coerce states without outright forcing them. Give states back their freedom to choose, a right which they had held for more than two centuries.