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.