OP - Having been married for over 4 years and being 26, DH (28) and I could probably walk into the college group, sans rings and changing to boyfriend/girlfriend, and no one would question our age; well until we opened our mouths, haha. (*ahem, yes...I'm also 22...*)
Having never been on a cruise and am not as aware of the atmosphere of activities and such, but I'm guessing that most people find friends when they are doing something of similar interest - whether its bingo, poolside drinks, dancing, Pilates, cooking, people watching, etc.
Also love the opinion that if I had a 18 year old daughter, I would not feel comfortable about her hanging around in a group with a 24 year old guy.