Good measures of frailty in age, and good correlations with mortality rate, have been comparatively easy for the medical establishment to discover and verify - take grip strength, for example. Here is another: "In an analysis of nine studies involving more than 34,000 people age 65 and older, faster walking speeds were associated with living longer: Predicted years of remaining life for each age and both sexes increased as gait-speed increased, with the most significant gains after age 75. In addition, researchers found that predicting survival based on gait speed was as accurate as predictions based on age, sex, chronic conditions, smoking history, blood pressure, body mass index and hospitalization. ... Walking is a reliable tool to measure well-being [because] it requires body support, timing and power, and places demands on the brain, spinal cord, muscles and joints, heart and lungs. Slowing down is associated with getting older. By age 80, gait speed is approximately 10% to 20% slower than in young adults. ... In the study, gait speed was calculated using distance in meters and seconds. All subjects were instructed to walk at their usual pace and from a standing start. Average rate of speed was 3 feet per second (about 2 miles an hour). During the 14-year course of the study, there were 17,528 deaths. Those who walked slower than 2 feet per second (about 1.36 miles an hour) had an increased risk of dying. Those who walked 3.3 feet per second (about 2.25 miles per hour) or faster survived longer than would be expected by age or sex alone." Eliminating the slow spiral down into frailty and physical incapacity is one of the noble goals of longevity science.