I think that with Princess if you are under 21 you have to have someone booked into your stateroom who is over 23 or 24 years old. Drinking age is 21. If you are under 21 your cruise card will have a hole punched into it, and the bartenders will refuse to serve you.
Princess does try to enforce this 18 year old drinking policy (too bad you can fight and die for your country but can't drink?) ... BUT, you will have no problem ordering drinks in ANY of the ports in the Caribbean. Princess does a good job in keeping their ships a comfortable place (even limit the number of children per cruise which is really good for the children as well because the "child/preteen" program is not overloaded) for all ages.
Just want you 18/19/20/21 year olds to know that some of us who are now in our 50's do understand your situation and know it does not seem fair ... to us either!!!!