Trying to stay healthy into old age is a better goal than attempting to live as long as possible. Of course, no matter how hard we try, most of us will still fall ill at some point in our lives, whether because of genes or just bad luck. And people with chronic diseases “can have extremely meaningful lives even as maybe their physical capacity declines,” said Dr. Deborah Kado, a professor of medicine at Stanford University and a director of the Stanford Center on Longevity. Perhaps even more vital than the number of years a person lives, in sickness or in health, is their attitude and the way they spend the time they do have. “How you decide to live, whatever your time horizon, is extremely important,” Dr. Kado said. “None of us have that much control, and so it’s really how do we address the challenges as they come at us.”