Toeing the start line of the Mashpee Superswim, Violet Birenbaum clocked she was one of the few teenagers gearing up for the one-mile race. “It wasn’t very competitive, but I noticed I was one of the few younger kids,” she said. “I was nervous about that.” At 12 years old, Birenbaum was the youngest competitor at the June 13 event in Mashpee. Still, the sixth-grader at Chenery Upper Elementary School held her own, swimming one mile in 32 minutes, 7…