I agree with Brad and Sandy, too. Let me throw another suggestion in, NCL
I'm in my 50s and my hubby's in his 60s, however, for the shorter cruises, all of these lines seem to have a great mix of age groups. They all offer good clubs/dancing/activities. We still enjoy having fun, and we've always had a good time on any of these lines. The shorter cruises tend to have younger crowds, so I think you'll be able to have a great time on any of them.
Let us know what you decide!