Mandatory retirement for faculty in the U.S. ended in January 1994, and the forecasts about its likely effects were, we can say in retrospect, rather Panglossian:
A 1991 study by the National Research Council, an arm of the academy, found that most tenured professors chose to retire before age 70, and, on many campuses, before 65.
The study predicted that those patterns would continue in the absence of mandatory retirement....
A recent CHE article asked, "Are there too many elderly professors?" As the piece notes,
In some fields, especially in the humanities, there’s no guarantee that earlier retirements will in fact result in greater opportunity for younger scholars. As Jonathan Zimmerman wrote a few years ago in our pages, “A mass retirement by senior professors wouldn’t derail the adjunctification train; if anything, it would speed the train up.” I have spoken to aging scholars who would in fact like to retire but are reluctant to do so because they know their tenure line will not be replaced.
At least at the major reseach departments, however, retirees do tend to be replaced.
How is lack of mandatory retirement affecting philosophy? Consider just the last two years: Warren Goldfarb (Harvard) is retiring the year he turns 75; Nathan Salmon (UC Santa Barbara) the year he turns 73; Frederick Neuhouser (Barnard/Columbia) the year he turns 67; Elliott Sober (Wisconsin) retired the year he turned 75; Richard Arneson (UCSD) the year he turned 78; John Carriero (UCLA) the year he turned 67; Stephen Daniel (Texas A&M) the year he turned 73; Paul Guyer (Brown) the year he turned 75; Charles Larmore (Brown) the year he turned 73; Peter Vallentyne (Missouri) the year he turned 71; Paul Woodruff (Texas) the year he turned 79. This isn't everyone at research universities who retired during this two-year period; I omitted some folks who retired for health reasons or because of retrenchments at their schools. Those who retired before 70 had compelling personal reasons for doing so.
But now consider faculty older than 75 who are still teaching at just a handful of the top philosophy PhD programs in the U.S.: at Princeton, John Burgess is 76 this year and Philip Pettit is 79; at NYU, Ned Block is 82 this year, Hartry Field is 78, and Kit Fine is 78; at Rutgers, Ernest Sosa is 84 and Stephen Stich is 81; at Pittsburgh, John McDowell is 82 and Mark Wilson is 77; at Yale, Stephen Darwall is 78; at UCLA, Tyler Burge is 78, David Kaplan is 91, and Calvin Normore is 76. Many of these philosophers continue to be highly productive and actively involved in teaching and doctoral supervision.
So how should we answer the CHE question in philosophy? I honestly don't know.