Just a thought, that might be of interest:
I believe the legal drinking age in most of that strange country across the Atlantic is a rediculously high 21 years old?
The rest of the world mostly only requires you to be 18, and some parts of the world have the age even lower. So... If the ship is sailing under a flag other than the USA, it is officially the sovereign territory of that nation and the laws of that nation apply while on board. Thus, the moment the ship enters international waters, the legal drinking age goes down to 18.
Now, of course, the Captain of the ship is legally responsible for everything that goes on, but he is answerable only to the maritime authorities of whatever nation his ship is flagged under. It is highly unlikely that those authorities - mostly concerned with such weighty matters as piracy, shipwrecks, oil spills, etc, would bother themselves about the trifling matter of a few under-age drinkers.
Thus, it is left down to the staff at the bars to determine whether they should serve an individual or not. And as this is supposed to be an adults only type cruise, with no kids... You get my point?