I've had to field this one before. I tell people the reason why more older adults dance than young ones, is that they've finally come to that point in their lives where they can do something for themselves that they want to do, instead of raise brats, cave to peer pressure and social stigma, and work hard, and they've finally realized that stumbling around a dance floor at some crappy bar with a bunch of other idiots isn't as fun as the people doing it think it is. They grew up, got some class, and now they can do something that the younger set, unless they're dancers, too, probably couldn't keep up with. Or in short, you could always say "Stupid people do seem to enjoy saying things like that" or maybe "To avoid having to run into jerks at bars and other places," if you're feeling particularly assy.