I just got off of a cruise on the Carnival Elation this morning and the doors to the staterooms were not opened until exactly 1:30pm. Check-in was fairly easy and started promptly at 11:15 after the VIP guests checked in.
I did not do any gambling in the casino, but I am sure that the minimum age is 21 for that. It was mentioned numerous times in the Fun Times newsletter about Texas Holdem' players meeting at a certain time and they also had a tournament.
Hope this helps. Good luck on your cruise!