There is nothing wrong with wanting to be on a cruise with people in your age group and interests.
I am single, I took a cruise to Alaska with my family and was not looking to meet anyone, though I did go to the dance club a couple times with another single lady. Good thing I wasn't looking for a date because there were no single interesting men on this cruise as far as I could find!
Took a cruise to Mexico the weekend after Thanksgiving on Carnival Paradise with family and met several interesting single men (and women) at the Meet and Greet in the piano bar the first night of the cruise! According to the Carnival Caper of events the Meet and Greet's were in different areas depending on your age, but at the piano bar there were lots of people there from 21 to 60ish, some single some not. I had a great time in the piano bar with Barry (the piano man), he encouraged people to mingle with those they did not know prior to the cruise, which made it easy and fun to meet new people. The adult comedy shows were really fun also, and I was able to attend the comedy shows with some of the single poeple I met at the piano bar which made them even more fun. By all means pick your cruise by who you think might be sailing, but make the most out of whatever cruise you choose, I did and loved them!