The boom in cycling has been largely down to the middle aged well heeled. That forms by far the largest demographic you see out on "Serious" bikes and wearing all the kit. The percentage of youngsters into cycling is tiny in comparison so when the older ones get fed up or grow too old there will be few coming through to take their place. The whole thing mirrors the motorcycle boom of the mid nineties which lasted for around fifteen years and then collapsed, taking a huge number of retailers with them. And the age demographic was exactly the same.
I am surprised at that, I agree some middle aged guys come back to cycling but never would have thought that made that much difference to bike sales overall. Obesity is another reason, obese parents are unfortunately encouraging the same in their children, very few obese people ride bikes, sadly.