This is an interesting question about a sensitive topic. Some people lose mental acuity as they age. Also, people get sick and that can affect how well they perform their jobs. Many federal judges have a great deal of experience, which is good, but the aging process and other factors can take their toll. So what happens?
1. It is well-known that there are informal efforts by federal judges to encourage a fellow judge to retire if the judge is losing his or her mental capacities. I don’t know about this in detail – – it happens behind closed doors. But I think we all have had personal experiences where, for example, we need to have a conversation with someone who is losing his ability to drive safely or to perform his job as well as he used to – – I assume the conversations among federal judges are similar.
2. Federal law provides judges with an option to resign because of a physical or mental disability.
The law also details a procedure for the President to appoint a new federal judge if a judge who is eligible to resign because of a physical or mental incapacity fails to do so. The procedure requires that a majority of judges in a Circuit’s judicial council first issue a “certificate of disability.” I’m not aware of any recent examples.
3. Another federal law allows older judges to enter into “Senior Status” where they take on a reduced work load. Many judges choose this option.