Returning to the young members issue: I do not have a solution, far from it, but I am a member of the Cambridge Astronomical Association.
The CAA has a very thriving and active membership, many of whom are youthful by the standards of other local societies, let alone the BAA. They appeal to primary schoolchildren as well as those a very few decades older.
One could do much worse than to contact the CAA committee to see how they manage it. Their website is at https://www.cambridgeastronomicalassociation.com/ and I could provide introductions to a few of their people on (off-forum) request.