Normally 21 they tend to follow the USA drinking laws but worth checking with the cruise line?
If it is a American ship 21 is the age for alcohol and they enforce the the law rigidl.
What cruise line? It is usually 21 and you could be thrown off for giving alcoholic beverages to minors.
Oh well, I won't bother then, plenty of other places in the world to see... thanks for the info..