At Harvard University, a 63-year-old retiree — trained in philosophy, but retired from real estate investing — teaches a classroom of her peers about postmodernism.

In Maine, legislators appropriate $150,000 to bolster courses for retirees at 15 ”lifelong learning institutes” from Portland to Presque Isle.

And on the campus of Lasell College in Newton, a four-year-old retirement ”village” requires residents to complete 450 hours of study each year, attracting international attention and a waiting list of more than 100 senior citizens.

These scenes from modern retirement feature a new class of knowledge-driven retirees, who are heading back to school in droves to revive old passions or to tackle new topics. And with the 65-plus population expected to double by 2050, specialists predict steady growth in senior college enrollments.

Nearly 30 years after Harvard and Duke University established two of the first institutes for older learners, the number of programs nationwide is close to 500 and climbing, say specialists. In the last three years, the California-based Bernard Osher Foundation has awarded grants to dozens of new senior colleges nationwide, including one at Brandeis University in Waltham. And Osher, a Maine native, plans to double the number of Osher institutes over time.

”People are living longer, they’re healthier, and they’re looking for ways to participate in life,” says Kali Lightfoot, director of the Osher Lifelong Learning Institute at the University of Southern Maine and national coordinator for all 50 Osher programs.

In Maine, where 6,000 students 55 and older are enrolled statewide, seniors pay a $25 annual membership fee and $25 to $50 per class, including books. At Harvard, retirees — who also teach courses — must apply to the 27-year-old Institute for Learning in Retirement, which maintains a maximum 500 members ages 55 to 95. They pay $325 per semester for courses including ”Love and War in Classical Japanese Literature” and ”Boston Architecture Since 1930,” director Leonie Gordon says.

A more informal approach is possible at Harvard Extension School, where 266 students over 60 are enrolled, says Michael Shinagel, dean of continuing education. World history and religion courses are among the most popular for seniors.

”When they started careers and families, they had to set things aside, and now in their 60s or 70s or 80s they’re saying, ‘I never did read Plato,’ or ‘I always wanted to learn Italian,’ ” he says.

Older students say they relish the freedom to focus on learning. As a young man, Marty Foley worked full time while he earned a business degree at Boston University. Now 66, the retired Dorchester florist is studying digital photography at the University of Massachusetts at Boston. ”You can’t serve two masters,” he says of his undergraduate days. ”It’s much easier now.”

The vibrant atmosphere on campus is also an attraction. After class, Foley says, he lingers over lunch to soak up conversation. ”I get excited over there,” he says.

UMass-Boston also benefits from the presence of the five-year-old lifelong learning program, director Wichian Rojanawon says: Many older students volunteer on campus, helping foreign students practice their English.

At the University of Southern Maine, senior students raised $100,000 to paint and furnish classrooms they use one day a week. ”We try to be good campus citizens,” says Lightfoot, the program director.

When Newton’s Lasell Village became the first retirement community in the country to require classroom study — the rule helped resolve a land-use battle by giving the project an educational purpose — no one knew how seniors would respond, says Paula Panchuck, the development’s academic dean. Four years later, the 200 residents average 540 hours of annual study, beyond the 450 required, under a flexible definition that includes fitness classes and time spent reading and writing.

Live-in learning doesn’t come cheap: Lasell residents, who must be 65 or older (half are couples and half are single) pay an entry fee of $300,000 to $750,000, 90 percent of which is returned when they die or leave. Monthly fees of $2,300 to $4,500 cover class costs, meals, maintenance, and housekeeping for their one- or two-bedroom apartments.

Bernice Kazis, 80, moved to the village from a three-story Swampscott home eight months ago, after her husband, a retired rabbi, died. With degrees in psychology and education and years of experience resettling Russian immigrants, Kazis says she is busy with new ideas. She’s reading Socrates for an art philosophy course called ”The Examining Life,” and she buzzes with excitement about her forensics class, where students examine slides of dead bodies and tackle tough questions such as King Tut’s cause of death.

”Somebody said, what are you going to do with [the education]? I’m not going to do anything with it, except stimulate my thinking,” she says. ”It’s a lot more relaxed, because it’s for me.”

Jenna Russell can be reached at jrussell@globe.com.http://www.boston.com/business/articles/2004/11/17/thirsty_for_learning_seniors_fill_college_programs/