It’s fair to say that the most important people around President Trump these days are his White House chief of staff John Kelly, his national security adviser John Bolton, his economic council chief Larry Kudlow and his outside legal counsel, Rudy Giuliani.

Those four top advisers come from quite different backgrounds, but here’s one thing they have in common: They are contemporaries of the 72-year-old president. The ages of Messrs. Kelly, Bolton, Kudlow and Giuliani are 68, 69, 70 and 74, respectively.

The...