He had become the local expert on what he called the “unwanted side effects of old age,” so Dr. Bob Ross, 75, rubbed arthritis cream onto his hands and walked into an exam room to see his seventh elderly patient of the day. He had been a doctor in the remote town of Ortonville, Minn., for nearly five decades, caring for most of its 2,000 residents as he aged alongside them. He delivered their children, performed their high school physicals, tended to their workplace injuries and now specialized in treating the wide-ranging symptoms of what it meant to grow old in America.