He is 73, and he takes the stage. He runs a hand through his shock of white hair, exposing strands of tendons attached to wiry but impressive biceps. His T-shirt stretches tight over a broad and lean chest. His jeans make his torso look more like an upside-down triangle than a rectangle. He slings the electric guitar to the front and, with aged hands, slams a chord that fills the arena.  His friend is already on stage. The friend is 75, and he sits at a piano. He is a bit thicker than the guitar player but still spry enough to pull off a few moves on stage. His hands…