In medical school I was taught that the incidence of chronic, disabling disorders, particularly Alzheimer's disease, increases inexorably with age. I therefore expected that people older than 95 years, often called the oldest old, would be my most debilitated patients. Yet when I became a fellow in geriatrics, I was surprised to find that the oldest old were often the most healthy and agile of the senior people under my care. In fact, the morning I was scheduled to interview a 100-year-old man as part of a research project, he told me we would have to delay the visit. He had seen 19 American presidents take office, and he would be busy that morning voting for the next one.