I agree with Becker that it would be good if universities (if everybody) were permitted to impose a mandatory retirement age on their employees. As a matter of theory, however, the removal in 1994 of the professorial exemption from the Age Discrimination in Employment Act's ban on mandatory retirement ages need not have affected the average age of retirement of professors. In general, a law that affects only one term in a contract should have little effect on behavior, because its effect can usually be nullified by a change in another term. Eliminating mandatory retirement age is a good example. If a university wants professors to retire at, say, age 65, it can pay them to do so; that is, it can buy out their tenure contract. In the long run, the professoriat itself will pay for the buyouts, at least in part, because the opportunity for a buyout is a valuable option for which professors will be willing to pay by accepting a somewhat lower wage. (See the discussion of mandatory retirement in chapter 13 of my 1995 book Aging and Old Age.)
Even if the result of abolishing mandatory retirement age is higher costs for universities, to the extent that all competing universities are affected, they will be able to shift most of the cost to students in the form of higher tuition. And to the extent that even generous buyouts are refused, universities can offset the effect by increased hiring of young faculty, albeit at increased cost. For just as higher energy costs need not alter the age mix of the faculty, neither need the abolition of mandatory retirement do so. Of course, this assumes that universities want a youthful faculty. As Becker points out, and I below, there is a good reason for universities to want to have a youthful faculty: young faculty tend to be more innovative.
The average age of professors has increased, but the increase may largely have resulted from factors unrelated to the abolition of mandatory retirement ages: namely, continued rather dramatic increases in the health and energy--the youthfulness--of the elderly (which may narrow the productivity gap with young faculty); lighter workloads in elite universities; and delegation of teaching to teaching assistants and non-tenure-track teachers, reducing the demand for tenure-track faculty and hence increasing the average age of tenured faculty.
The political divergence between old and young faculty (the older being more leftwing) is at first glance odd. If the adoption of a political ideology is driven by information, then since the information available to young and old is the same there should be no age-related difference in ideology. It is plausible that the young would be drawn to more extreme positions, whether left or right, on the political spectrum because lack of experience would make them more susceptible to radical schemes. But in academia it seems that Marxist and other extreme positions are more commonly embraced by the old than by the young.
I doubt that the adoption of a political ideology is normally a result of a rational weighing of information. I think it is more commonly a matter of temperament interacting with aspects of personal identity (such as race and sex), life experiences, and nonrational beliefs, such as religious beliefs. (I argue this in my recently published book How Judges Think.) This makes ideology resistant to change based on new information. The expansion of the universities in the 1960s, together with the waning of antisemitism in university admissions and faculty appointments, resulted in a large influx of Jews, and Jews, for reasons never adequately explained, are disproportionately left-leaning. In addition, the expansion must have lowered the age of faculty, and for the further reason that teaching provided a refuge from the draft during the Vietnam War.
The extreme to which the youth of the 1960s was drawn was leftist, and the left in the 1960s was farther to the left than today's left. If, therefore, ideology is largely resistant to information, there will be a tendency for a person's ideological identity to persist notwithstanding events, such as the collapse of the Soviet Union and the rise of free-market ideology, that might be expected to move a "rational" ideologue rightward.
Becker rightly points to the danger that an increased age of university faculty members will result in reduced innovation. But this cannot be seen as an automatic or inevitable consequence of the age discrimination law even apart from the theoretical argument that I began with, because, assuming an inverse correlation between productivity and age, universities can lower the age profile of their faculty without violating the law and probably without even having to expand the faculty. The age-discrimination law applies equally to private businesses, but one does not hear it argued that there are too many old employees in private firms. Universities could abandon tenure and adopt performance-based compensation schemes. In addition, they could reduce the possibly too methodologically conservative influence of older faculty by reducing the role of faculty in appointing new faculty members.