I consider myself as a quality single, but I guess not everyone has the same definition.
I have cruised twice with singlescruise.com in the past 3 years, and I'm currently 35. I found plenty of early thirties people to hang out with out of the hundreds that were with the group. I even made some over friends over 45 (gasp!). hee hee
I don't go looking to hook up, or meet the love of my life, or even the lust of my life. I book with a group because most of my friends are married with children, or don't want to save up for a cruise and I want to know I have people to hang out with. I went on my first trip solo and had a total blast, met tons of cool people, and loved that I could just hang out alone too if I wanted it.
I think the quality of the cruisers and the quality of your cruise depends on your outlook. There were just as many people younger, older, thinner, taller, prettier, funnier as there were people who were half as witty, half as interesting, and 75% less fun to hang around with then me!
I have found that those who love to cruise and a fun bunch, and just like in land-life you find people you click with and people you don't. I hope you give it a try and remember to go with an open mind.