Robinson Jr. and Melton Jr. in The Atlantic:
Old age, said the ancient Greek philosopher Bion of Borysthenes, is “the harbour of all ills; at least they all take refuge there.” While the quotation is obviously nothing new, some things do change, among these society’s pace of life and its knowledge of aging. In the past two centuries we’ve seen more change in these areas than in the law designed to protect America from the dangers of elderly presidents. Given the current crop of presidential contenders, the law’s failure to keep up may very well erupt into both a political and a constitutional crisis in the not-so-distant future. The original Constitution did provide for presidents who grew unable to carry out the duties of the office. Under Article II, Section 1, in the event of a president’s “inability to discharge the powers and duties” of the presidency, the vice president would take over. But this short passage skirts over several pitfalls. One of the biggest was first raised in the 1787 Constitutional Convention by John Dickinson, a delegate from Delaware. Addressing an early draft of the provision, which had slightly different wording, he asked two questions that still plague the country today: “What is the extent of the term ‘disability,’” he asked, “& who is to be the judge of it?” So far as anyone knows, his questions went unanswered by his fellow delegates.
Presidential inability hasn’t exactly been a rarity. Beginning with George Washington, who nearly died of a monthlong bout of pneumonia a few months into his presidency, a number of chief executives have been incapacitated, from periods ranging from a few hours to several weeks. In 1881, President James A. Garfield, mortally wounded by an assassin, lingered for two and a half months before dying. In 1919, Woodrow Wilson suffered a major stroke, which permanently altered his cognitive functioning. During his final years, Franklin D. Roosevelt was suffering badly enough from cardiovascular disease to cause his top subordinates severe worry before his death, from a cerebral hemorrhage, and in 1981 Ronald Reagan was under general anesthesia for a time following John Hinckley’s assassination attempt. There are many more examples.
More here.