Oh lordy lordy, this is not the Titanic. This Legend of the Seas is something you would only dream about. And i believe the table tennis facilities are fabulous. After all, that is a young persons game so you and your boyfriend should be fine.
Your best bet would probably be Ocean Village so long as you are happy with a casual approach to cruising and don't want to dress up formally in the evenings. This isn't to say their won't be any older people on there because there will be. I suggest you look at their web site or get a brochure to see what they have to offer.
It all depends what/how you consider "old" to be honest. We have been on 5 caribbean cruises with RCL, Princess and Carnival.
I am 51 and my wife is 49 and we have as much energy and do everything younger people would do on cruises and have a really good time and a great laugh with all fellow cruisers, mainly - there is the odd misery, but we just ignore them. We get on really well with youngsters - in their 20's, as we have 2 children in their 20's. The Carnival ships would probably suit you I would say overall, as they are the "Fun" ships; but TBH it really doesn't matter what the other people are like, it really is about making the cruise enjoyable and your own outlook.
My advice then is to try a few cruise lines, if you can afford it, and make the most of what they offer and you won't even notice other people mainly!
Fine food, several bars, great shows, daytime beatiful beaches, sailaways at sunset, great service... what people?
All the best anyway.