I ride 30km per day back and forth from home to college. I'm 43, I ride an old steel bike (ca. 20kg). I do this out of necessity, since it's the cheapest and most convenient way to travel around the city I live [1]; but I also do it out of love for the joy it provides me. Granted that the city's overall elevation helps a lot (it's almost plain here, but many people still think I'm crazy for this choice of transportation).
As others have pointed out, weather is only a bummer when you're not properly clothed (I'm over-generalizing here, but you get the picture). It also helps to live on a place where the average temperature never goes down below 24ºC [2]. On the down side, it rains a lot here. When I say a lot, a do mean a lot.
Bicycles are fun. I couldn't imagine myself living without one.