I don't want to discourage you or anything, but my mom and her boyfriend (both in their 40's) went on Ruby last November. They said that there wasn't any night life & the ship was deserted after 10pm, the service or food wasn't that great, and that it was like being on a "floating nursing home." They also mentioned that they felt like they were some of the youngest people on the ship.
However, my mom makes her own fun wherever she goes & still managed to have an alright time.
Sorry this isn't the best, but I just figured I'd let you know their experience.
__________________ Future Cruises ❤
Carnival Elation - November 25 & November 30, B2B
Past Cruises ❤
Navigator of the Seas - 2005, 2007, 2009, 2010
Mariner of the Seas - 2007
Liberty of the Seas - 2008
Carnival Liberty - 2009, 2010, 2011, 2013 B2B
Carnival Miracle - April & June 2011
Carnival Dream - 2011, 2012
Carnival Glory - June & September 2012
Carnival Imagination - 2013
I don't know about Ruby Princess specifically, but I've been on two cruises since I've been two old for youth programs, both on Carnival Miracle. Participate in stuff during the day also, not just at night.
Now cruiseaddict said she heard the ship is full of old people, so I don't know how well, if at all, my advice will work. I've only ever been on Carnival since I turned 18, and sometimes it seems like the party on Carnival starts after the lifeboat drill, and doesn't end until the cruise is over.
Past Cruises (all except one from NYC)
Carnival Triumph to Canada: August 1999
Carnival Victory to Canada: July 2000
Celebrity Zenith to Bermuda: July 2001 AND July 2002
Celebrity Galaxy to Caribbean (from Baltimore): July 2003
Voyager of the Seas to Caribbean: July 2004
Crown Princess to Caribbean: July 2006
Carnival Victory to Canada: August 2007
Carnival Miracle to Caribbean: May 2008
Staten Island Ferry, I mean what else in NYC is free?
I am 18 as well and will be on this cruise. I also know tons of young people going on it. I would assume the cruise would not be full of old people during late December ,as it is Christmas break and everyone is taking time off, especially all students.