The question then becomes: Is being president more like having to do a lot of math problems, or more like having to contain one’s emotions and make difficult decisions? It might be both: Presidents need to have a spry brain, capable of assimilating new information and rapidly adding it to their cognitive repertoire. But the job is, most crucially, about making decisions—extremely difficult decisions that are, unlike arithmetic, matters of judgment and value. The rightness of a decision is often unknowable ex ante. In these treacherous exercises, the elderly do not do badly, and impetuous youngsters sometimes come very close to getting us all killed. When you travel to a country with lots of road deaths, you should choose an old taxi driver with a very slow car.
There is also the question of energy, which—unless Sanders (78) and Biden (77) really are on amphetamines—seems certain to decline, and probably within the four- or eight-year period during which they might hold office. (Compared with them, Trump is a spring chicken—a Rhode Island Red, judging by color—of 73.) The job of a president has, historically, been exhausting emotionally and physically, and we should prepare for the possibility of a president who, like Ronald Reagan in his later years in office, simply slows down.
This, too, might have some upsides. The more presidents slow down, the more decisions get made by other people. Call it the “Weekend at Bernie’s presidency,” though either Democratic candidate, or Trump, could graduate to this geriatric mode of administration. And perhaps we’d all be better served if other people—and not Biden, Sanders, or Trump—were making decisions. I see ample reason to question the soundness of the judgment of all three men.