t judges assent to not long after reaching that age. As explained in the linked material:
Financial concerns were paramount for judges who retired after senior
status, as well as for judges who retired directly from active
service, according to the survey results presented on Sunday. “I
wanted more income” was the most popular reason cited for retirement
for both groups. . . .
Federal judges can retire or take senior status, with the option of a
reduced workload, if they satisfy the “rule of 80.” The requirement
says they must be age 65 or older, and their combined age and years of
experience must total 80. It is an attractive option because their pay
remains the same, but their taxes fall because they don’t have to pay
taxes for Social Security or Medicare.
In other words, if judges retire once meeting the "rule of 80" they can get the same pay with less taxes without doing any work, and can take senior judge assignments in the discretion of the chief judge of their court for additional pay, while vacating their position. If a chief judge feels that a senior judge is no longer intellectually able to do the work, the chief judge can decline to assign a senior judge to more cases.
Only at the U.S. Supreme Court level is the power associated with holding the post so great relative to financial considerations, that the compensation system incentives don't work, and it is arguable that in the case, for example, of Justice O'Connor, that the compensation factor was an important reason for her resignation.
The concern is also mitigated at the U.S. Supreme Court level, and even at the Court of Appeals level, because those judges mostly make decisions as part of a panel of judges, rather than individually. So, if the other judges on the panel make the right decision, the incompetent judge's decision doesn't matter.
For trial court judges, the prospect of appellate review of their decisions partially mitigates the risk of a judge making bad decisions due to declining intellectual capacity.
State and Local Age Limits And Disability Rules
Most states have chosen not to follow the federal example with regard to judges and have either imposed a constitutional retirement age, as in the case of Colorado, where Colorado Constitution, Article IV, Section 23(1) requires judges to retire at age 72, and/or have a commission overseeing judges (and sometimes other civil servants) that can require them to retire due to disabilities which often arise from old age.
Term limits, which are common at the state and local government level, also often impose de facto age limits, because most very elderly elected officials mandated to be so old by being re-elected many times after being first elected at a much younger age. Term limits limit the extent to which this can happen.