You may have heard that justices and other federal judges enjoy “life tenure” — something that is easy to believe when the average age of the Supreme Court justices is 69. However, Article III of the Constitution says only that federal judges, both of the Supreme Court and of lower courts, can retain their offices as long as they maintain “good behavior.”

This seems to imply that the justices have a duty to retire when they are no longer fit to work full time. That duty is a rule in some countries: Britain, for instance, forces judges to retire at 70.

It is said that Supreme Court justices used to direct a junior colleague to notify a dotty justice that it was time to resign. But as the influence of the court expanded in the second half of the 20th century, that practice seemed to disappear. Some justices, even those seriously unfit, have held on to their awesome power and status long beyond what was reasonable. William Rehnquist, who continued to work on cases in 2005 even as he was dying of cancer, is the most recent example. The celebrated Thurgood Marshall, who was 82 when he retired in 1991, was another.