In colonial America, generally speaking, there were no purchase ages, and alcohol consumption by young teenagers was common, even in taverns.
[1] In post-Revolutionary America, such freedom gradually reduced due to religious sentiments (as embodied in the temperance movement) and a growing recognition in the medical community about the dangers of alcohol.
From 1976 to 1983, several states voluntarily raised their purchase ages to 19 (or, less commonly, 20 or 21), in part to combat drunk driving fatalities.
By mid-1988, all 50 states and the District of Columbia had raised their purchase ages to 21 (but not Puerto Rico, Guam, or the Virgin Islands, see Additional Notes below).
[6] 1980:19[30] *For an established religious purpose; *When a person under twenty-one years of age is accompanied by a parent, spouse, or legal guardian twenty-one years of age or older; *For medical purposes when purchased as an over the counter medication, or when prescribed or administered by a licensed physician, pharmacist, dentist, nurse, hospital, or medical institution; *In a private residence, which shall include a residential dwelling and up to twenty contiguous acres, on which the dwelling is located, owned by the same person who owns the dwelling; *The sale, handling, transport, or service in dispensing of any alcoholic beverage pursuant to lawful ownership of an establishment or to lawful employment of a person under twenty-one years of age by a duly licensed manufacturer, wholesaler, or retailer of beverage alcohol.