One of the harsh realities that sports have thrust into the public eye is the rise of intergenerational rivalry. The same medical advances that permit aging boomers to hold onto public offices, academic sinecures, and big houses for longer than ever allow older athletes to dominate to an extent that would have been unthinkable mere decades ago. When a pro turned 30 in the early 2000s, it usually signaled the beginning of the end. Yet, from 39-year-old Lionel Messi’s run to the World Cup finals to Tom Brady winning a Super Bowl at 43, early retirement is no longer a...