Pete,
You don't see Young single commuters in USA?
I would of thought the opposite, that younger single people can commute easier.
I commute half-time. Married, 47, 2 kids.
I commuted up to 22yrs old and started again when 42. I likely would of commuted off and on those years if Mtn bikes would of been around.
I don't look at bike commuting and bike riding for training/fun as compatible. At one time i did but now if i want to have fun on a bike i don't want to do the congested, boring, commute to work.
Sure i could do some variations but would rather ride where i want , come home and shower and then either bike or drive to work. I bike to work for ethical reasons now and save money. Maybe if there was a shower at work i would combine the two more.
Erik