The document is a PDF file available at http://www.nhtsa.dot.gov/people/injury/alcohol/Community%20Guides%20HTML/PDFs/Public_App7.pdf which is found at the National Highway Traffic Safety Administration.
From the document:
What is the national age 21 drinking law?
The National Minimum Drinking 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. The U.S. Department of Transportation has determined that all states are in compliance with this act.