At port, off the ship, she would need to follow that country's laws. So whatever country you're in if she's of drinking age she would be able to drink there.
On the ship (most sailings) no drinking until 21 and she is not allowed to bring anything on board. I.e. if you stop in nassau and go to a liquor store she is not allowed to carry it on board to be checked.
Also word of caution as someone who sailed when she was 20 with her parents. Pro tip: don't let her "steal" or get anything from you or someone else on the ship. Doesn't end well.....trust me on that one LOL