Originally Posted by Herzlos

I think money and time are big factors - younger folk are more likely to be working physical jobs and anyone who has retired likely has lots of time. So I think it's mostly a case of boomers are more likely to be able to buy a bike and find the time to ride said bike than any other factors.

Most of the cyclists I see here are middle aged towards elderly men on leisure rides, but I'm quite rural. In the wider urban areas it's a much better mix.
I do see a few kids on MTBs occasionally though and bikes are still popular Christmas presents.

It could just be that it's hidden - you don't really see the boomers who don't move from their couch because they never move from the couch*, yet you see more millenials/X/Y's out and about but on busses or walking or whatever.

*Anecdotally, my boomer parents wouldn't walk to their car if they could avoid it, whereas I'd never dream of a short drive if it was avoidable.