I'm 30, not athletic at all and daily ride my bike uphill 6 km to work in 22 minutes.
Here in Holland most normal bike commutes are often quite a bit longer than 5 km, and everyone from businessmen to pregnant women seem to manage fine...
In the summer, I end up carrying around a towel and an extra shirt. It's not so bad.
Obviously, these aren't the only reasons cycling isn't as popular in the US. There are certainly US cities which are just as flat and mild.
I can walk (no problems there and I like walking) or drive, for me there's no in between.
The problem is that bike lanes are often not maintained during winter. Luckily they are in my town, at least to some degree.
Cold can be a problem, but it's also a problem for drivers. Rain can be a problem. Heat can be a problem. But all of these can be overcome, and these are just some of the problems with getting more people to stop driving to work (I personally think proximity to work and perceptions of being seen biking to work are bigger problems in the USA, but I haven't studied the problem much).
Some key thing are that it's very flat and there are bike roads which are snow plowed consistently. And there are lots of students.
[In Minneapolis, nothing stops bicyclists — not even winter](http://www.latimes.com/nation/la-na-minneapolis-winter-bikin...)
[Biking is up, even — and especially — in cold weather](http://www.startribune.com/local/minneapolis/236827801.html)
[Twin Cities winter bike community feels the pinch](http://www.startribune.com/lifestyle/247300181.html)
[Why would you bike commute in the winter](http://www.mprnews.org/story/2014/01/29/why-would-you-bike-c...)
[Ride on in Winter (2011)](http://www.startribune.com/lifestyle/114216374.html)
Perhaps I'm severely overestimating the shape people are in, but I had a job where I commuted 15km by bike. You can ride 15kph without breaking a sweat, so that's an hour at a relaxed pace and probably 30-40 minutes at an athletic pace.
I'd guess that the median American can ride 10km (6.2 miles) with no issues. I could be wrong, though.