"A person's mind is like a knife, it needs to be sharpened. If you don't use your mind, then it gets dull." -- Nien Cheng, 72, author of Life and Death in Shanghai

WHEN CHENG finished writing her best-selling Life and Death in Shanghai two years ago, she said to herself, " 'What are you going to do? You haven't got a project.' I could easily have spent my time doing nothing, but I believe all my life that we should always read and think about problems, and take part in discussions."

She joined American University's Institute for Learning in Retirement, one of 100 such programs at colleges, universities and community colleges nationwide specifically designed for senior citizens.

Cheng participated in two study groups her first year, "The Bible as Literature" and "The Twentieth Century American Novel," taught by other retired members of the Institute. "Through reading Faulkner and other writers of the '40s and '50s, I got to know something about American life," she says. "It's effortless study, because the story is so interesting. And, of course, there is the opportunity of meeting other people of similar interests and age groups. In the course of chatting, we exchange our life's experiences. They all bring a different background, and I contribute a little too. It widens one's horizons."

Programs like AU's Institute have proliferated in the last 10 years in response to the increasing proportion of older people in the population -- one out of eight Americans was over 65 in 1984. As the traditional college population of young adults declines, colleges are also looking to "non-traditional" groups like seniors to fill classrooms.

"We feel this is a movement that's just getting into stride," says Dick Cortright, a senior education specialist for the American Association of Retired Persons.

Senior citizens can pick from three types of programs. Those like AU's are based on one begun by a group of retired teachers at the New School of Social Research in New York 25 years ago. Retired people join college-sponsored institutions or learning centers, where they plan their own programs, and lead seminars, classes and study groups themselves. There may also be opportunities to do independent research and present papers. Usually members pay a yearly fee, $200 at AU, which allows them to attend several study groups and to audit courses in other parts of the university.

Other programs like "the Maturals" at Northern Virginia Community College, are run by colleges themselves. Courses, which tend to be more practical or recreational than the peer membership programs, are planned specifically for seniors and taught by paid or volunteer faculty, sometimes off campus to reach more people. Last spring, Maturals offered courses on subjects ranging from history to auto mechanics under the Maturals program.

Finally, some universities simply give older people reduced or free tuition on regular courses. Tuition at public institutions in both Virginia and Maryland, for example, is free to people over 65.

The demand for all three types of programs is great. Nearly 900 students over 60 took credit courses at NOVA last year, including three who are 87 years old. Brooklyn College's Institute for Retired Professionals is the nation's largest peer program, with 2,100 members choosing among 80 classes each semester. It opened in 1977 with only 157 members.

Last year Montgomery College drew almost 6,000 older students, and Prince George's Community College more than 5,000, by holding most classes off campus at senior centers, nursing homes, churches and high-rise apartment buildings. "It's for convenience," explains Betty Whittaker, coordinator of Prince George s Programs for Special Populations. "They might not take our classes if they had to take transportation to campus."

Programs are as varied as the colleges which sponsor them, and each has its individual character and audience. Virginia Commonwealth University's "Free University for Senior Citizens" is the cheapest of the peer programs, with eight-week courses held at a church and a Jewish Community Center, no registration required and no charge. "Nobody comes unless they want to learn, and if they don't like the course they don't come. You get very honest reactions to what you're doing," says Eugenia Williams, the program's coordinator.

A Fraternity of Learning

PEER LEARNING-teaching groups attract seniors who have had lively, often distinguished careers, though not all of them have college degrees.

"The one thing our members have in common is that they've all been intellectually active, and they want to be with people both inside and outside the classroom who are curious, read widely and love to discuss ideas," says Kenneth Young, director of AU's Institute. "They can not get at a senior center or country club what they get from our kind of program."

Begun by retired lawyer Jack Blume in 1982, the Institute currently runs 45 to 50 study groups each term. Last spring some members studied "China Today," led by a retired Foreign Service specialist on East Asia, a China analyst for the CIA and a missionary while others discussed four plays with a former film actress.

UCLA s Plato Society (Perpetual Learning and Teaching Organization), begun in 1980, is one of the most academic senior programs. Its 285 members are interviewed before acceptance and each is responsible for research and presentations in the study groups. Members are encouraged to learn about topics new to them. "If you've been a banker all your life and you take nothing but economics, you're not stretching your mind at all," says Adriane Lorin, the Society's program administrator.

Harvard's 375-member Institute for Learning in Retirement is also a strong academic program encouraging discussion and research. Begun in 1977, it is also one of the oldest, and 20 of its 90 charter members have not missed more than one semester in 10 years. "They are a very committed group," says Carol Risting, the Institute's director. "Six years ago an 88-year-old member was supposed to deliver the paper in class, and we had a severe snowstorm. Everything was closed but she made it, in case someone else showed up. They did, too."

Eckerd College's Academy of Senior Professionals in St. Petersburg, Florida, emphasizes intergenerational as well as peer learning. "In our society we tend increasingly to segregate our retired people," said Leo Nussbaum, recently retired director of the Academy. "One of our objectives is to figure out ways of reducing the chasm between generations, to enrich the educational program for both groups."

In one particularly successful course a regular faculty and an Academy member jointly lead undergraduates and seniors through an on-site course in Greece. After such experiences, Academy members often become counselors, mentors and friends to younger students, according to Nussbaum.

Academy members can choose to live in a nursing home-apartment complex or planned condominiums on college land, and can audit Eckerd's year-long required courses, "Western Heritage" and "Judeo-Christian Perspectives."

"You play the role of teacher, of devil's advocate, of expert witness," explained retired Major General Joseph Pedirtz, an Academy member since 1984. "We studied Eli Wiesel's Night, and during World War II, I happened to have been with the unit that helped liberate Dachau and could talk about it. You can add that dimension to the classroom."

Pedirtz, 66, joined the Academy for the "intellectual challenge," and for the variety of ways the program involves him in the college community. He's worked on fundraising, taught spot courses on living overseas, and led a 12-part series on the Civil War for Academy members with a retired history professor. "I take on two or three projects a year, plus whatever comes along on an ad hoc basis," he says. " The rest of the time, I go out and listen to lectures, debates, forums on various subjects. That's the lure of it. You take what you want, you give what you want, when you want to do it."

Perfect Time to Study

OLDER PEOPLE attending college look for this kind of flexibility; they want freedom from grades, tests and requirements and prefer daytime scheduling and shorter courses. They also seek programs where they meet others the same age. "Our members really cherish the social aspect," said Harvard's Risting. "After retirement their friendship circle changes, often their families move away, and, as they mature, they are besieged with death. Here they can make new friends and build some very lasting relationships."

Seniors also differ as a group from other students. "They have the perspective that comes from experience, having to carry responsibility and be autonomous for a long time," said Judith Green, a professor of philosophy at Eckerd. They make the perfect liberal arts students, enquiring, pursuing enrichment, with leisure to study.

"The educational experience of most undergraduates comes at a time when they're not really ready for it," says Eckerd President Peter Armacost. "They don't understand the value of it, they're too focused on career preparation. Ten years later, they're ready to deal with the great issues in Western thought, but they don't have the time. What we're finding is that many of our Academy members are throwing themselves with vitality into an agenda that's been theirs for a long time."

Dottie Brazelton, 69, had such an agenda, but like many seniors never attended college because of the Depression. "I was offered a free home with the president of one of the south Georgia colleges if I could bring $35 for my first quarter tuition," she says. That $35 was not to be found."

After marriage, work and raising three sons and several foster children, she started taaking free courses at NOVA three years ago, "so I could write up all the family stories, lifetime experiences, memories of my grandparents for my children and grandchildren in a way they would not be ashamed of," says Brazelton. Her first year she made straight A's and her short story won first prize in the Loudon County campus literary competition. Last year she began tutoring other students at the college's Writing Center.

"I am achieving the dream I had since I was a child," concludes Brazelton. "I am going to college, I am studying, I am teaching. And I love it!"

Linda Barrett Osborne writes frequently on health and education.