Seems a bit dubious...

From an article in the NYT 19FEB07:

"Alcohol sales on airlines are not regulated by the Federal Aviation Administration, which requires only that alcohol be served by the airline itself, not another party. According to the Air Transport Association, a trade organization that represents passenger carriers, state law binds airlines only when they are on the ground. As a result, some carriers, like American Airlines, will buy liquor licenses in states that mandate them for airplanes, like New Mexico. But others, like Frontier, will not."

(The entire article, about a drunk driver heading home after a flight, is here: http://www.nytimes.com/2007/02/19/us/19liquor.html)

This does nothing to answer the question, though. There is no federal law, and state/local laws apply only on the ground. What law do they follow in the air?

Incorrect; there is no federal law mandating a minimum drinking age, but there is a law that addresses it: the National Minimum Drinking Age Act of 1984, which withholds some federal funding for states that do not legislate and enforce 21 as the minimum legal drinking age. Thus, while the federal gov't does not mandate the minimum age, it nevertheless does address it.