23 U.S.C. § 158 — National minimum drinking age

(a) Withholding of Funds for Noncompliance. (1) In general. (A) Fiscal years before 2012: If a state allows people under 21 to legally buy or publicly possess alcohol, the Secretary must withhold 10 percent of certain highway funds due to that state. This applies starting on the first day of each fiscal year after the second fiscal year following September 30, 1985. (B) Fiscal year 2012 and thereafter: Starting in fiscal year 2012, the amount withheld from a noncompliant state changes to 8 percent of the funds apportioned under section 104(b), paragraphs (1) and (2). (2) State grandfather law as complying: A state counts as compliant, even without a strict law, if it already had a law (in place by October 1, 1986, or shortly after its legislature next met) that banned buying and publicly possessing alcohol for anyone under 21 — as long as that law didn't take away rights from people who were already 18 or older and could legally drink before the law changed. (b) Effect of Withholding of Funds: Any highway funds withheld under this section after September 30, 1988, are gone for good — the state can never get that money later. (c) Alcoholic Beverage Defined: This section's rules cover three things as "alcoholic beverage": (1) beer, as the Internal Revenue Code defines it; (2) wine that is at least one-half of one percent alcohol by volume; and (3) distilled spirits, as the Internal Revenue Code defines them.