In the good old days universities had a hard retirement policy that once you reached age 65 you were retired whether you liked it or not. Then in the age of entitlement it was declared that this was discrimination on the basis of age and thus could not be allowed. Universities bemoaned the fact that they had no firm financial projections under the new policy, and many different policies were introduced partly to solve this problem. In some cases you could gradually go to half-time, and then at some age to quarter time, until you eventually did retire, but most of these policies were voluntary.
It is useful to look at the broad picture that these changes produced in the university community. If there was indeed some general plan of development in a particular discipline like zoology, committees could lay out a future hiring plan but it was usually chaos because the time frame was so uncertain. So in my experience most carefully thought out hiring plans went out the window and hiring became ad hoc with the accompanying ‘departmental drift’. So, as a hypothetical example, if a professor in entomology retired, he or she might get replaced by a young assistant professor in microbial genetics.
On a larger scale, we need to look carefully at the consequences of keeping older professors on the books commandeering relatively large salaries. There are no clear rules but in general one might recognize professors that are worn out at age 55 and ought to retire, others that are happy to stop at 65 and relax more, and others who ask to stay on indefinitely. Every case is an individual one. Some of the age 55 scientists are still vigorous and any concerned department ought to work to make their life easier so they can continue to work. Others of the same age should be encouraged to go. The same should occur at age 65. The worry I have most is about those over 65. I give no names but I can list brilliant scientists who continued to be paid and work until they were 75 or 80. I can also list scientists who were brilliant in their time but had passed the gate by 65 (or even 55) but insisted in taking up a position for many years after age 65. This is a tragedy for the individual, for the department, and in particular for young scientists looking for a university position but finding none because the money is tied up in professors well past their use-by-date. I would expect that the only possible solution to this issue is for the university to evaluate every professor over 55 with strict demands of performance if they wish to remain on the payroll and to do this on a 1 or 2 year timetable. No one likes doing such evaluations so perhaps the university would have to hire one of the many hard-nosed CEOs of companies that are seen to be effective at firing all their workers.
None of this is to say that any and all professors who have retired at age 55, 65, 75, or 85 should not be encouraged to continue research work, but they must do it on their retirement savings. In my youth I met a 98 year old Drosophila researcher who was continuing to do valuable research in his long retirement. In Canada the federal research agencies do not seem to care how old you are when they evaluate the quality of your research work and contributions to science, so they at least do not appear to discriminate in awarding research funds on the basis of age. Scientific journals do not ask you how old you are when you submit a potential scientific paper.
There has always been a paradigm that scientific advances are made entirely by young scientists, so that, as the joke goes, almost all mathematicians should be shot after age 30 (that is a joke….). In at least some of the ecological sciences this age paradigm is not correct, but nevertheless I think it is morally recumbent on older professors to realize that their time on the payroll should be limited in order to release funds for the aspiring young scientists who can rejuvenate university departments.