@oaktonarcher,
Well the drinking age for a long time was 21 as well as the voting age. When the voting age was dropped because of the Vietnam war (if you can die for you country you should be able to vote idea) so was the drinking age. But as of this time, it was totally state to state.
Until in 1984 when the Federal government stepped in. in 1984 the United States passed the minimum drinking age act, which required all states to raise their drinking ages to 21 or their government highway funding would be cut.
but as Ken brought up, there are a lot of exceptions... here are a few of them
An established religious purpose, when accompanied by a parent, spouse or legal guardian age 21 or older
Medical purposes when prescribed or administered by a licensed physician, pharmacist, dentist, nurse, hospital or medical institution
In private clubs or establishments
In the course of lawful employment by a duly licensed manufacturer, wholesaler or retailer.” 1
Many of the states that have chosen to specifically prohibit alcohol consumption by those under age 21 have a variety of exceptions. For example,
Some States allow an exception for consumption when a family member consents and/or is present. States vary widely in terms of which relatives may consent or must be present for this exception to apply and in what circumstances the exception applies. Sometimes a reference is made simply to "family" or "family member" without further elaboration.
Some States allow an exception for consumption on private property. States vary in the extent of the private property exception which may extend to all private locations, private residences only, or in the home of a parent or guardian only. In some jurisdictions, the location exception is conditional on the presence and/or consent of the parent, legal guardian, or legal-age spouse.
Some States also allow exceptions for educational purposes (e.g., students in culinary schools), religious purposes (e.g., sacramental use of alcoholic beverages), or medical purposes. 2
http://www2.potsdam.edu/hansondj/LegalDrinkingAge.html for more info