Is the 21 minimum drinking age on a cruise ship a law, or just a policy that the cruise ship adopted

I was told that international law states that the drinking age in international waters is 18, but I was reading in the cruise liners' websites that you have to be 21 to drink aboard the ship. Is there some law saying this, or is 21-and-over policy for alcoholic beverages merely a policy that the cruise liner has adopted??
on international waters you cant get in trouble for underage drinking - just like Mexico - i dont think you even have to be 18 to drink - the ships probally adopted this policy to safe guard themselves - now you can get in trouble for consuming alcohol in the states being under 21 but i dint think you can for consuming it once in international waters -

drink responsibly
home girl against alcoholic homicide

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