Point taken re age groups- at the Eight Bells in Tenterden we've seen several young performers come through & learn about session audiences. Thing is that they then disappear for several years, either because they are good enough to make the grade or because careers & families take over. My feeling is that they, like several other performers now surfacing, will be back when they feel the need to rediscover their roots.
Clubs like Nellies at the Ivy House in Tonbridge can survive on audiences of around 25-30, but only if guests are prepared to chance a hat-take. Organisers can't really be blamed if they choose not to subsidise on a regular basis. (A familiar story here- ideal room at previous venue, then Ceroc moved in at full volume in the next room. Only Doug Hudson could compete!)
Personally I prefer the club format, which gives a performer the chance to build a set/ contrast material etc- but if the audience doesn't come we have to adapt. Most attenders are players these days, certainly in West Kent. So back to the question- how can a folk club generate a reliable audience?