It is an age thing. There are not so many clubs any more because young people as a rule don't want to go to folk clubs (or more importantly set them up), they want to do other things, like-if you can believe it-form clubs! There are plenty of open-mic nights and events organised by young people around dancy things (Ross McKinlay from Bedlam does DJ/live music nights in venues around Matlock), but, as folk clubs were set up by people older than them for their own friends and social circles, young people sometimes have trouble fitting in and have started to do their own thing.
When I started in folk clubs I felt like the only young person there a lot of the time, especially performing where you feel like you are putting on a show for your parents' proud friends-like plays you did when you were little around the dinner table-and rather than conform to what was already there, people have started up other stuff. It certainly isn't as heady and focused as it was in the 60s, but that doesn't mean that it isn't happening. The big youth boom at the moment is in ceilidh dance music anyway, which unfortunately is inappropriate in most pub back-rooms.