CAMBRIDGE -- This fall, professor Roy J. Glauber regaled a dozen Harvard freshmen with tales of his youth -- as an 18-year-old undergraduate helping to develop the atomic bomb on the Manhattan Project.
That was more than 60 years ago. Since then, Glauber has raised two children as a single father, became a Nobel laureate, and in September, celebrated his 81st birthday. He has no plans to retire.
He is part of a wave of professors changing the face of academia by working into their 70s and occasionally even their 80s, particularly at the nation's top universities. A law that went into effect in 1994 banned the common university practice of requiring professors to retire at a certain age, usually 70. Improved health and longevity have also encouraged many to stay in the laboratory and the classroom.
This year, 9.2 percent of tenured professors in Harvard's Faculty of Arts and Sciences are 70 or older, compared with none in 1992. Other universities have seen jumps in the percentage of older professors, although the actual number remains small on many campuses.
The graying of university faculty has stirred vigorous debate. Some in higher education, including Glauber's students, say many older professors are brilliant researchers and riveting teachers with plenty to offer.
But some academic leaders say the abundance of older professors is plugging the pipeline, making it harder to hire young faculty members and bring fresh ideas into labs and the classroom. At the worst, they worry that the perseverance of older professors will crowd the young out of scholarly professions altogether.
"The aging of the faculty, caused in large part by the absence of mandatory retirement, is one of the profound problems facing the American research university," said Lawrence H. Summers , who as Harvard president pushed for the hiring and tenure of more younger scholars. "It defies belief that the best way to advance creative thought, to educate the young, or to choose the next generation of faculty members is to have a tenured faculty with more people over 70 than under 40, and over 60 than under 50."
Harvard's Faculty of Arts and Sciences, which includes the undergraduate college and the doctoral programs, has 186 tenured professors age 60 or above, and 156 under age 50.
Nationally, the percentage of full-time faculty members age 70 or above has gone up three-fold since 1995, but remains 2.1 percent, far lower than at Harvard and many major universities.
Some economists say that the phenomenon of elderly professors is not a major concern at the majority of universities and that the aging of the baby boom generation contributes most to the graying of the faculty.
However, the growth in the proportion of professors 70 and older is marked at elite private universities, including many of Boston's largest institutions. Specialists say that's probably because the jobs are more desirable and the teaching loads lighter than in other types of schools.
"Most of us are pretty motivated, we wouldn't have gotten where we are if we were not because there are so many hurdles to clear," said Glauber, referring to the challenges of winning tenure and research grants. "That motivation doesn't suddenly turn off at 65 or any other age you can decree in advance."
Glauber entered Harvard at age 16, in 1941. He spent a few years at Los Alamos and in other research positions, but settled in at Harvard for good in 1952. He's now a grandfather of five, and last year won the Nobel Prize in physics for his 1963 work on the behavior of light. His findings contributed to numerous inventions, including laser instruments.
His drive to continue his research remains so great that he says he routinely works till 2 a.m. or 4 a.m.
A theoretical physicist, he doesn't have a lab or graduate students working with him, but he said his research would suffer profoundly if he wasn't surrounded by his colleagues, because science is such a collaborative effort.
"Anybody who works by himself is not very likely to have his views broadened," he said.
Glauber, who teaches a regular load for his department, one class per semester, said his students were "a real source of inspiration."
Lisa Eliana Gingold , a student in Glauber's fall freshman seminar, "The Atomic Nucleus on the World Stage," said it was meaningful to have a teacher who helped shape the history of the topic. She said he could discuss the latest issues in physics with ease.
Glauber said he wasn't concerned about occupying a slot that could be used to hire a younger professor, in part because Harvard's faculty has grown substantially in recent years.
William C. Kirby , who stepped down as Harvard's arts and sciences dean last summer, raised the issue several times in letters to the faculty. Kirby, who is still on the Harvard faculty, told colleagues that improving gender diversity of the faculty requires bringing in more younger professors.
"Every university is concerned about their capacity to bring in good young talent early in their careers in sufficient numbers," he said in an interview.
Other researchers say that the lengthening careers of older faculty makes it harder for younger scholars to build their careers because fewer positions are available. Scientists are the least likely to retire because most can't do research without their university-based labs, said several professors.
The slower pace of retirements in the sciences is one of the reasons that young scientists are spending more years in low-paid postdoctoral fellowships and the average age for a scientist to get his or her first major research grants from the National Institutes of Health has risen from 37 to 42 in the last 25 years.
A few schools have tried various policies to encourage faculty to retire, but it's not clear how well they work. Columbia University offers generous financial incentives, yet 9.5 percent of its tenured faculty this year is over 70 .
Harvard Business School officials could not provide data on the age of their faculty, but say the business school's retirement incentives work . While they relinquish their faculty slot, retiring faculty get access to a suite of offices with secretarial help, and access to travel and research funds.
Those who retire in their mid-60s are eligible for some compensation to make up for lost income. Professors can be rehired on renewable one-year contracts.
Dwight B. Crane, 69, retired 1 1/2 years ago. He teaches half-time and uses the rest of his time for a consulting practice and golfing vacations with his wife.
Crane said he wanted to retire while he had enough energy for other projects. And, he said, "I didn't want people to wonder why I was still here . . . . No matter what your profession is, when you get older you begin to lose a step."
Marcella Bombardieri can be reached at bombardieri @globe.com.