At nearly 83, DH is still working, not in an office, not full time but as part of a team of engineering consultants. He works from home about 2 - 4 hours a week. He worked nearly full time for the first 5 years after he retired and then has gradually wound down. He stopped site visits at 70, He has months when there is no work and he obviously did not work when he was seriously ill. He is part of a team, so if he suddenly started producing bizarre reports or calculations, it would immediately be noticed.
I think age and fitness to work is a very variable thing. We continually hear reports that Trump's behaviour mirrors one specific form of dementia and there is a refusal to talk about medical problems he certainly has.
Looking around at friends, family and other older people I see. The older people are the more variable they are in health, stamina and mental acuity. I have had friends succumb to dementia in their early 70s, In his early 90s my DF still held key roles in three local groups. He wanted to resign but no one was prepared to take over from him.
I do think for roles in government or key public organisations, public, private and state, there should be age cut offs, say 80 with discrete medical and mental tests for those over 70 before appointing them to senior posts, and regular tests while in post.
As more people grow old more healthily, and there are fewer young people, older people are going to have to continue to play their part in life by working or volunteering, but there needs to be an acceptance that this will come with regular mental and physical monitoring.