The federal Bureau of Labor Statistics reports that the fastest-growing segment of the workforce is people 75 and older. Shirley Martin is one of them. When we met her five years ago, she was a greeter at Walmart, making slightly more than minimum wage. Today, she’s 77 and manages a group home for adults with disabilities in South Dallas. She’s better off financially, but to stay that way, she works full time — and she has no plans to stop.