None of this has anything to do with repertoire or what songs are being sung or how they are being sung.

It's simply that someone in their 20s is unlikely to want to keep returning to a club where everyone else is 70+

Basically, the only way you can attract young people to a club is by having young people in the audience at that club. Vicious circle. So in my opinion, the only way septuagenarian/octagenarian folk clubs can attract younger attendees would be to have regular 'takeover nights' - where they invite young, proactive singers to curate the night, promote it. Be honest with them - let them know what it's about.

The main singaround I go to - called 'Folk of the Round Table' - is by some margin the youngest average age of any folk event I've ever been to. Everyone there is in their 20s and 30s apart from a small handful of middle aged people (including myself).

Here's a video that gives you a peek inside the session, featuring clips of singers and the crowd:
https://www.youtube.com/watch?v=Ue3ekdBh-FI

The main reason it attracts young people is because it is run by young people - at a venue associated with young people.

Bridget & Kitty are the main organisers (two singing sisters in their mid-20s). Quite a lot of Goldsmiths art college students go along too (Matchstick Piehouse is in New Cross). The venue has a lot of LGBTQ+ nights, drag nights, jazz/funk jams, indie band nights, poetry nights. Quite often young people turn up to the venue not even knowing it's a folk night, they're there just cos they know the venue always has something interesting going on.

I've seen young people regularly turn up who know nothing about folk music, who - 6 weeks later - stand up and sing a song they've learned off, say, a Lankum or Stick In the Wheel album. In fact, it's brought home to me how much influence Lankum are having: that act is a frequent source of material for 20something singers.