Keep in mind that older cyclists tend to be retired, children have moved out and may have downsized their homes.
Not having to cook for a brood, read bedtime stories, cut grass and commute to a 9-5 job means lots of time to ride miles, train hard and take the all important recovery naps.
At 56 I'm just starting to get there. My children are either off to college or independent. I accrue tons of vacation time and my career is at a point where my schedule can be flexible. I'm in better shape than some of my 35 and 40 year old friends and acquaintances who have small children and just purchased homes. They can't seem to find time to train or even just ride with any consistency.
Bragging? Well, last year I did a 110 mile ride @ 17.7 MPH on a 48x16 fixed gear. I also rode an 18.4 MPH century on a geared bike.
-Tim-