Several geriatricians say that procedures that two decades ago were seldom considered for people in their 90s are now increasingly commonplace. They include hip and knee replacement, cataract surgery, heart valve replacement, bypass operations, pacemaker implantation and treatment for slow-growing cancers. With such rapid growth of centenarians, debate has mounted over how far to go—as well as how much Medicare money to spend—in providing major medical services to extend already very long lives.