The Harvard Medical School's New England Centenarian Study has a Web site focusing on persons 100 or more years old—"the fastest growing segment of our population." Now 40,000 strong, the population of U.S. centenarians could increase to 3 million or more within the next decade. At present, roughly 90 percent of them are women. Find out where the oldest of the old live and ponder new observations on what distinguishes those reaching the century mark.