I used to work for Carnival so I can tell you the exact truth.
DISCO= Some ships allow guests 16 and up into the disco... and some are 18 and up. It just depends on the ship. NEVER has it ever been 21 to enter the disco on a cruise ship.
CASINO: 18+ Not 21. Yes, the do card if you look under 18.
ADULT COMEDY SHOWS: 18+ not 21.
DRINKING: 21+ HOWEVER, if a parent/guardian were to buy their son/daughter a drink (fo fo drink, beer.. whatever) than that seems to be ok, But you must be 21 to buy an alcholic drink.
Therefore, if your Son/daughter is 18 or up (16 in some instances), they can do EVERYTHING but buy drinks.
Hope this helps y'all...
Have a good one,