It is true that Carnival attracts a younger group of singles, especially on the shorter cruises. However, I have found on most organized singles cruises that there are always a few in their 20s and a few over 60. It is a good mix, and the idea is to have a good time. I don't see where age makes a difference when it comes to cruising. But, hey, I live in South Florida where most people don't dare buy green bananas so I don't worry about mingling with all ages.
And I agree with cruising with a group, because these days you are going to find huge families traveling together because of discounted group rates, and that means fewer singles to hang out with...unless you are with a group.