Depends where you are, although it seems harder than ever to get and keep rifles, I am a member of two pheasant syndicates. Neither of them are really wanting for potential members. There's one where the membership is, on average in their early 60's and the other where it'll be more like 45-ish. Both have youngsters that come and one of them has a few in their 20's. So it's not all doom and gloom. The main problem we have is the city-centric parliament that less and less have country dwellers in it.
But having said that the average age of gun users is pretty old, you have to take into account that it's something that you might have been involved with as a kid, but went away from as a young adult when jobs, lurve, kids, house, etc. takes up all your time and money. Then, in middle age, when financial and time pressures ease a little and the kids have flown the nest, it's often something you return to.