the wikipedia discussion of this is accurate though a bit confusing. the punchline? no where in the US can anyone under 21 legally purchase alcoholic beverages.
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"Financial incentives create de facto federal purchase age of 21. The National Minimum Drinking Age Act of 1984 states that revenue will be withheld from states that allow the purchase of alcohol to anyone under the age of 21. Some states do not allow those under the legal drinking age to be present in liquor stores or in bars (usually, the difference between a bar and a restaurant is whether food is being served). Contrary to popular belief, since National Minimum Drinking Age Act of 1984, few states specifically prohibit minors' consumption of alcohol in private settings (an exception includes Connecticut). In some cases or states, alcohol permits can be purchased at a cost to the parent or legal guardian. As of 2006, 20 states do not specifically ban underage consumption and an additional 15 states have family member and/or location exceptions to their underage consumption laws.
Federal law explicitly provides for religious, medical, employment and private club possession exceptions; as of 2005, 31 states have family member and/or location exceptions to their underage possession laws.
Underage purchase of alcohol, though illegal in all fifty states, is not a federal offense... " (that is, the federal government does not enforce these laws, it is left to local & state authorities)
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http://en.wikipedia.org/wiki/Legal_drinking_age#The_Americas