Prohibition was a time in history when the legal drinking age was determined?
Not really. During National Prohibition, the minimum drinking age was essentially zero.
The drinking age on an airplane is determined by the country to which the aircraft is registered. Even though you may be legal, the airline may refuse to sell to passengers under a certain age. If the aircraft is registered to Canada for example, the legal drinking age is 19, if the aircraft is registered to the united states, the legal drinking age is 21.