The history of this subject can be found in James F. Mosher, "The History of Youthful Drinking Laws: implications for current policy" in "Minimum Drinking Age Laws: an evaluation" (ed. Henry Wechsler, Lexington, Mass., 1980) pp. 11-38. The earliest such regulation that I have noted was passed in New York in 1770, forbidding tavern-keepers to sell or give liquor to a person under the age of sixteen who became intoxicated as a result; I am pretty sure that Mosher also mentions other such regulations, some of which may predate this one. An important point that Mosher makes is that pre-Prohibition minimum drinking age laws were intended to protect children from being exploited by drinks sellers, which is very different from the present position. I would strongly recommend reading his article. The subject is also discussed in detail - and here comes another puff - in my forthcoming book "Drink: a social history of America".