Ian,
It's an all too common story when it comes to forums these day - as the older members give up on the hobby which a forum is based around, they're not being replaced by younger members because it's so much easier to just post a question on Facebook, even if it has been asked a million times before, rather than join and search a forum that's overflowing with knowledge.
From my late teenage years, I took over ownership of a large model-specific automotive forum, which was also IPS based. I kept it alive for about 7 years following the ownership change, but I took the wheel right as Facebook introduced their Groups feature, and it slowly went downhill from there.
It was only a few years ago that I finally gave up on it. It was overdue for its passing.
Fortunately, I recently volunteered to take over the technical running of another, larger automotive forum (on vBulletin this time around), which is showing no signs of slowing down. However, I'm under no illusions that it will eventually suffer the same fate.
It's just the way that the world is going - the newer generations don't have the attention span for a forum. All you can do is try to keep the existing member base interested.
Cheers,
Ben