This is a good question - whether cruising is good for single cruisers.
1. The thing about Carnival being more of a "party" ship is partly true, but not the age thing. It attracts "partiers" of all ages.
2. That being said, no cruise is as much of a "singles party scene" as a lot of people seem to think. Would you go to Disney World for four days looking to meet people and make friends? Cruises are not that much different, the vast majority of people are with their family and friends and not looking to meet new people.
So, it doesn't matter that much which ship you take, because you could find a lot of people on either one, or not, just depending on your luck of the draw.
If you just want to get away and relax, not party too much, then either one is good. I would say the crew is friendlier on Carnival, but only when they are serving you, not when its time to go ashore, etc.
I suggest you re-ask this question in the Singles Forums
here and see what other singles have to say.