Cruise Critic has a forum for single cruisers.

If you decide to use a large cruiseline--e.g., RCCL, Carnival--you can ask the maitre'd to try to seat you at a table grouped with people close to your age.

First night of our cruise, Mrs. Swede and I (both in our late 20's) were seated at a 10-person table. The other eight were all in their 70's! After dinner, we asked to switch tables, and we got seated with a wonderful group of people (ages 24-42)--ourselves and another couple were married, two couples who both got engaged during the cruise, and two singles.