Going on Senior Status

Because the U.S. Constitution gives federal court judges lifetime appointments, and because implicitly this means Congress cannot set a mandatory retirement age for federal court judges, some judges work well beyond the normal retirement age. However, many of these judges do not work full-time, rather they go on "senior status." A recent Boston Globe article explains:

The senior status arrangement, enjoyed by some 500 federal judges around the country, allows older judges to go into semiretirement while mentoring the fresher faces on the bench and helping to clear the court’s cases. 

Federal judges at all three levels can take advantage of this status, however, when a Supreme Court Justice retires, they may only serve as lower court judges under their senior status. Once the judge takes senior status, s/he effectively goes into retirement allowing the President to appoint a new judge, with the consent of the Senate, of course.