My biggest concern about Biden’s age is if he dies after being elected and before the electors are chosen. 

You’d get a reasonable-sounding argument that the person who came in second should be president, enough to kick up dirt. 

And then it becomes a political debate over whether Trump or Harris should be president. The media goes nuts, red states go Trump, blue states go Harris, purple states end in chaos. SCOTUS intervenes or lets House decide.