Full US Federal Court Judges, known as Article III judges are appointed for life under Article III of the US Constitution. In practice this rule was found difficult because it meant that many judges stayed on the bench well after they were able to hear cases or handle a full caseload. The solution was to this is to allow judges who had a certain number of years service on the bench and who met certain age requirements to retire on full pay – or alternately, to take a semi-retired roll as “senior judges.”
When a judge takes senior status this is supposed to “open up” a judgeship for a new, younger appointee. Thus, in principle, District judges on Senior Status are supposed to be able to take a smaller number of cases – however, in practice, because of political wrangling over judicial appointments, many senior status district judges continue to handle a large caseload.
A judge becomes eligible for senior status after 15 years service at age 65, with the require service falling by one year per year of age to age 70.