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".