As ageing staff stay longer in the job, US universities find it hard to bring in young academics, says Jon Marcus.
US universities, concerned about a shortage of staff when those hired four decades ago retired, are facing the opposite problem: staff who do not want to leave.
Faculty taken on during the vast expansion of higher education in the 1960s and 1970s, many of whom are now in their seventies and eighties, are reluctant to take retirement.
Young academics are becoming frustrated. Universities, too, while acknowledging the value of experience, are worried not only about continuing to pay the considerably higher salaries commanded by many older employees but also about keeping their faculties fresh and dynamic in an increasingly competitive market.
The percentage of full-time faculty aged 70 years or older has tripled in the past ten years. A third of US faculty are now aged 55 or older, compared with fewer than a quarter in 1989, according to the Higher Education Research Institute at the University of California at Los Angeles.
The proportion of those aged under 45 years has fallen from 41 per cent to 34 per cent in the same period.
Elite universities have an even greater number of ageing faculty. At the Massachusetts Institute of Technology, per cent of those in mathematics and 6.4 per cent overall are aged over 70.
At Harvard University, it is more than 9 per cent, while at Columbia University it is nearly 10 per cent.
This is a result of the 1967 Age Discrimination in Employment Act, which banned most US employers from setting a mandatory retirement age.
Universities were given an exemption in 1986 allowing them to force professors to retire at 70, but the exemption was not renewed and it expired.
The lack of a standard retirement age means universities "will not be able to hire the number of young people that a vibrant institution must hire to stay at the forefront", concluded a committee at Johns Hopkins University.
The ageing of faculty at the school, the committee said, would lead to a "concomitant loss of flexibility in Hopkins's academic programmes".
Some schools have introduced incentives for older academics to leave.
Columbia, for example, offers medical and dental coverage for life and continued access to university facilities for faculty who step down.
A former MIT president has come up with the controversial idea of "rolling tenure", under which tenure protection would end at a certain age and would be extended in two or five-year increments.
The phenomenon is not confined to the prestigious universities. In the past five years, the number of faculty aged over 50 has increased by nearly 200 at Iowa State University, and the universities of Iowa and Northern Iowa combined. Over this period the total number of faculty hired has increased by only 100 across the three institutions.
But, according to David Ward, president of the American Council on Education and former chancellor of the University of Wisconsin, the issue of ageing faculty has not yet reached crisis point.
He said: "We're dealing with expanding enrolments, at least in the short run, so universities don't have a great incentive to unload faculty. If a few people do stay on and they're very good, there's no harm in that because the demand is there."
Faculty who know they cannot teach are likely leave voluntarily. Dr Ward added: "There's nothing worse than staying on when you have no fire in your belly. There's a kind of self-respect that kicks in, and then I think people step aside."
But there is little universities can do under tenure rules and discrimination laws if staff do not want to leave. Dr Ward said: "You cannot force people to retire."