http://epw.senate.gov/title23.pdf
§ 158. National minimum drinking age
(a) WITHHOLDING OF FUNDS FOR NONCOMPLIANCE.
(1) IN GENERAL.The Secretary shall withhold 10 per centum
of the amount required to be apportioned to any State
under each of sections 104(b)(1), 104(b)(3), and 104(b)(4) of this
title on the first day of each fiscal year after the second fiscal
year beginning after September 30, 1985, in which the purchase
or public possession in such State of any alcoholic beverage
by a person who is less than twenty-one years of age is
lawful.