The uncomfortable collision of a perilous pandemic and a boisterous battle for the presidency conveys an unwelcome but unmistakable message to aging baby boomers. The generation that has dominated pop culture and politics for nearly a half-century, that has loomed in the national consciousness as perpetually pre-eminent, may not be so ageless and invincible after all.

Joe Biden (who’ll turn 78 before Inauguration Day) and Donald J. Trump (who’ll be 74), would be the two oldest people ever nominated for the presidency by a...