I would suggest that you check out Singlescruise.com ,I have been on 2 of them before and I had a Blast and I'm 49 . There are all ages that go on these cruises as matter in Fact I'm going on the April 28 singles cruise and I can't wait because friends I made on the Last 2 singles cruises are going on this cruise . I enjoyed hanging around 35-55 age group . I hope this gives you a Pretty good Senerio.
I have taken ten cruises, all with Princess, and have had no problem whatsoever meeting folks and making new friends. Some itineraries or sailings might draw more single passengers than others, but I have not figured out how to predict which ones will go which way.
I have booked the HAL Amsterdam's Wayfarer Southern Carribean cruise Dec 1. Participating in the Share partner program and my TA has me at a table with other singles. It probably doesn't matter which cruise you take solo, it's how you handle it and what you do to meet other folks, be they single or not.