The mature years. Adult education. RETIREMENT

SINCE Durward G. Hall moved in at Eckerd College in St. Petersburg, Fla., he often finds himself poring over textbooks into the early-morning hours once reserved for sleep. Now his leisure time is more likely to be spent in the library than on a boat on the nearby Gulf of Mexico - once his favorite retreat.

These adjustments come more easily for Dr. Hall than for other Eckerd ``freshmen.'' You see, the retired surgeon and former six-term Missouri congressman has been on the college route before, and he wouldn't have chosen to live on a campus in his retirement years if he didn't love the academic life.

Retiring to live on or near a college campus is gaining popularity among many other senior citizens, who are looking for pursuits other than golf and pottery classes in their later years.

And institutions of higher learning, often aided by private developers, are putting out the welcome mat for older people, whose numbers are growing at a time when the traditional college-age population is dwindling.

Though retiree homes and college campuses may seem incompatible to some, others feel it's a natural marriage.

``The kinds of things that go on in universities are the kinds of things retired people like to spend their time doing,'' observes James Eden, a former college professor, who now is general manager of Marriott Corporation's senior-lodging division.

Marriott plans to break ground in early 1989 on a retirement residence on the campus of the University of Virginia in Charlottesville. At the same time, the university will develop a specialized academic program for senior citizens. Interest in the Marriott project is high. Mr. Eden says the company has already received more than 750 deposits.

Retirees find that campus life offers them access to an array of free or low-cost activities - cultural events, top-rate libraries and art galleries, athletic facilities - with which to fill their leisure time. An added advantage is that living costs are often lower in a college town.

In the midst of one of Florida's top retirement areas, a residential college program for senior citizens was a natural choice for Eckerd College officials. They felt that having older people on campus would benefit the younger students, the college, and, of course, the retirees themselves.

Eckerd launched its Academy of Senior Professionals in 1982. But the dream of an academic haven for retired people was not complete until the 1986 opening of College Harbor, a continuing care facility developed by a private firm on Eckerd property.

``We are people who have been busy and try to keep busy'' is how Loyd E. Grimes, 83 and a retired education official with the United States State Department, describes Eckerd's retirees.

``We get to these so-called golden years, and what are you going to do? You can play golf all day, but some of us would like to stay up with world affairs.''

As a resident of College Harbor and a founding member of the Academy of Senior Professionals at Eckerd College, Dr. Grimes says he's able to keep his ``intellectual curiosity'' satisfied.

Not only can members of the program audit college courses for $15 and use all Eckerd facilities free, but they have their own extensive program of activities.

They are offered lectures by such well-known speakers as newsman David Brinkley, as well as member-initiated study and discussion groups, opportunities to aid instructors at the college, and travel-study trips.

Those who wish to enter the academy undergo a thorough application process, much as if applying for college. The study program is aimed at those who have been high achievers in their working life - although this does not mean they must have a college, or even a high school diploma.

The main qualification is having ``a mind that wants to remain stimulated and a career of high achievement,'' explains Dr. Arthur L. Peterson, director of the Eckerd program. And people who desire such mental stimulation find that ``living on campus is special. It facilitates heavy involvement in the college. For some, this campus has just become home.''

His move to Eckerd ``has filled a gap in my life,'' says Hall, a member who spends half the year at College Harbor and the other half in his home state.

``Mrs. Hall has to stop me from studying after midnight to keep up with the students.''

While Eckerd's retirement residence is aimed at academically oriented retirees, other colleges have opened residences marketed mainly for retired staff and alumni. Some have found, however, that retirees who have no affiliation with the college are eager to take part in a campus life style.

For example, in 1983 the Meadowood Retirement Community at Indiana University opened mainly as a home for retired staff and alumni.

But when Bloomington made the top ten list in Rand McNally's 1987 Retirement Places Rated Guide, the on-campus retirement center - the only retirement residence in town - got national exposure, says Randy Hornstein, executive director.

``People didn't realize that this type of community existed,'' Mr. Hornstein says.

Residents of the 184-unit complex have all the standard amenities of meals, an attached nursing home, and transportation services. They also are offered university-sponsored classes on such topics as Shakespeare, the presidency, Indiana folklore, and great books.

As America's changing demography hits home, many more colleges can be expected to take interest in programs that bring older people to live on campus.

For example, officials from Cedar Crest College in Allentown, Pa., have visited Eckerd College and are considering complementing the school's Learning in Retirement program with a retirement residence.

And even colleges that don't offer on-campus residences for older people may find retirement homes springing up at their outskirts.

While chilly Ann Arbor, Mich., hardly seems the typical retiree haven, it has received plaudits as a retirement community in books and magazines.

The main attraction, all agree, is the University of Michigan, with its many cultural and academic offerings.

``There are a lot of them going up around Ann Arbor, and their market is the alumni returning to campus,'' says Joel Berger, director of the Alumni Enrichment Program at the university.

The enrichment program has some 1,300 participants, a number increasing by about 100 a year, says Mr. Berger.

Many of those who have retired to the Ann Arbor area feel the same as Gertrude Heubner, who is a Michigan alumna, along her husband George, a retired research director at Chrysler.

``There is so much to do in a college town. You get all the good plays from New York, you get all the orchestras, and it's less expensive on a retiree's budget,'' says Mrs. Heubner. ``It's a cornucopia of riches.''