If you want a sustainable community, then you need new members to counteract the inevitable loss of existing members. To get new members, you first need to help

If you want a sustainable community, then you need new members to counteract the inevitable loss of existing members. To get new members, you first need to help them find your community somehow, then you need to give them a reason to stay, then you need to have a process where the old guard can pass the torch to the new. For "helping them find your community", that means that you need to not be precious about the platform that you're currently on; you need to be where young people are. That means somehow convincing the old guard to be willing to change platforms to accommodate the possibility of new members. In practice this probably just means forking your community onto every new platform and convincing a sustainable core of experienced users to populate the new venue, leaving the rest behind; but forking small communities often means death as they lose the critical mass of users necessary for long-term appeal. For "giving them a reason to stay", that means that your community has to be welcoming to new users. But also, since new users will probably be young-ish, the topic of the community itself needs to be something that young people have a reason to care about. If you're a community of Victrola and vintage phonograph enthusiasts, I'm afraid you're not going to appeal to the youth no matter what you do; if your topic isn't perennially cool, then you might have already lost the battle for longevity. For "having a process to pass the torch", this means you need at least a little bit of a formal structure. If you have a forum, then who runs it, who sets the rules, who enforces the rules? If the answer is "one random guy", and then that random guy gets run over by a zeppelin, then you've lost. Basically, creating sustainable, long-lasting, intergenerational communities is extremely difficult. Anything that you come up with to solve the problem is going to start looking like either a religion or a bureaucracy.