The last time the nation held a debate with the presidency on the line, a candidate with about eight decades of life behind him faced the challenge of proving that he was still up to the job of running the country. He failed. Two and a half months later, the cast of characters has shifted and another candidate heading toward the octogenarian club confronts his own test to demonstrate that he has not diminished with age. Whether he passes that test may influence who will be the next occupant of the Oval Office. At 78, former President Donald J. Trump...