The following cruise lines require that all passengers in a cabin are 21 or older or that at least one person in the cabin is 25 or older:

Carnival Cruise Lines
Costa Cruise Lines
Holland America

The following cruise lines require that at least one person in each cabin is 21 or older:

Celebrity Cruise Lines
MSC Cruises
Norwegian Cruise Lines
Princess Cruises Lines
Royal Caribbean Cruise Lines

The following cruise lines require that all passengers in a cabin are 18 or older or that at least one person in the cabin is 21 or older:

Cunard Cruise Lines
Disney Cruise Lines

Gambling on cruise lines 18+

Carnival Cruise Lines
Holland America Cruise Lines
Norwegian Cruise Lines'
Celebrity Cruise Lines ( 21+ for Alaska Cruises)
Royal Caribbean Cruise Lines ( 21+ for Alaska Cruises)

Gambling on cruise lines 21+

Cunard Cruise Lines
MSC Cruises Cruise Lines
Princess Cruise Lines

Gambling on cruise lines 25+

Costa Cruise Lines