Here is directly from the Carnival Website and this applies to all ships:
Drinking Alcohol On Board -
The minimum age for the purchase and/or consumption of alcoholic beverages in the bars, lounges and gift shops is 21 years of age. Carnival reserves the right to refuse the sale of alcoholic beverages to anyone. In the event that Bar/Restaurant/Gift Shop Staff are in question that a guest is less than 21 years old, they shall request picture identification, prior to serving the drink or selling the bottle of liquor.
Guest must be 18 years of age or older to play the tables and the slot machines. Guests under 18 years of age are not permitted in the casino.
For those parents out there who think that Carnival will not serve your little darling, because they are under age, please be aware that they will be served, because not all bars are strict. Please caution your kids.
But you won't "purchase" the drink if you're under age. I've seen them on many occasions swipe under age sail-n-signs and the software must have some sorta alert on it. The bartenders pulled the drinks immediately and didn't appreciate (rightly so) bein put to the bother.