This is not an area where one can generalise, it has to be on a case by case basis that includes both a competency check and a medical.
The competency check can be carried out by the employer but must be witnessed by an independent and qualified representative of the licensing authority who has the final say, no 'old pals' act there.
I am still fit enough to fly safely, I know people several years younger than me who are not, there can be no situation of 'one case fits all'.