President Trump is 73, and if re-elected would leave the White House at 78, the oldest president in U.S. history.

The front-runner to challenge him is Joe Biden, 77. His rivals include Bernie Sanders (78), Elizabeth Warren (70) and Michael Bloomberg (77). Absent a Buttigieg breakthrough, the next president will be at least 75 by Election Day 2024; Mr. Biden will be 81.

Mr....