When Mitch McConnell, the Senate's 81-year-old Republican leader, recently froze up during a news conference, he triggered tremors of trepidation in both parties. If something like that were to afflict 80-year-old Joe Biden or 77-year-old Donald Trump, the whole trajectory of the campaign could be altered in an instant.
But even if those elderly leaders avoid such blatant examples of frailty, the M…