You don't stop running because you get old. You get old because…

The Dipsea Demon made it a motto, John J. Kelley made it a tradition by barely slowing during decades of marathon, and the New York Times reports it's now corroborated by German researchers:

A few years ago researchers at the German Sports University Cologne took a close look at the finishing times of 400,000 marathon and half-marathon runners between the ages of 20 and 79. They found no relevant differences in the finishing times of people between the ages of 20 and 50. The times for runners between 50 and 69 slowed only by 2.6 to 4.4 percent per decade. "Older athletes are able to maintain a high degree of physiological plasticity late into life," the researchers wrote.