During the next decade, the Washington area's 22 colleges and universities, a major industry employing more than 55,000 people, will face a dramatic drop in their traditional constituency, the 18- to 22-year-olds. To keep classrooms filled, the institutions are turning increasingly to the adult market and its vast potential for part-time students.

"Universities are at a crossroads," says Thomas Coffey, dean of continuing education at American University. "They can choose to deal with traditional students and see their enrollments diminish. Or they can reach out to attract adults."

In the metropolitan area, as many as 300,000 students, ranging in age from 17 to 80, will enter classrooms this year. Area colleges will be trying to attract some of these students to their campuses in a business where the stakes are already high and the competition fierce.

This competition drive is in sharp contrast to the post-World War II era of higher education.

Since the end of the war, colleges and universities throughout the country have ridden an unprecendented wave of expansion, driven by an ever-increasing crop of 18-year-old.

But that wave will crest next year when the number of 18-year-old American peaks at 4.3 million. During the 1980s, the number of 18-year-olds will plummet 21 percent to a total of 3.4 million by 1990.

Already, public school systems are feeling that decrease. As elementary schools and now junior highs are closed for lack of students, college deans and presidents are becoming painfully aware that unless they find a way to fill the gap, they face the same fate.

Their response has been a massive retooling of programs to emphasize adult education and lure older students back to campus, in effect, to replace 18-year-olds with 40-year-old.

These efforts are part of a trend that is reshaping the mission and direction of American higher education in the last quarter of the 20th century.

"Our emphasis will shift more toward paying attention to the needs of people who want continuing or advanced education," says John S. Toll, president of the University of Maryland. "Ever since World War II, the universities have been expanding in response to a growing demand of high school graduates. In the future, our goal will be increasing our contribution to the social and economic development of the region and the state."

"More and more people are coming to grips now with lifelong learning as a concept," says Howard Greer, dean of community services at Montgomery College. "They're getting used to taking courses almost routinely."

In the Washington area, at least, this concept of life-long education, is seen as the economic life-blood of area universities.

Consider:

The 300,000 students expected to enroll in higher education courses here will attend classes at 22 metropolitan colleges, the Smithsonian Institution, the U.S. Department of Agriculture Graduate School or branches here of two dozen colleges from outside the area.

Those students will provide full-time employment for at least 55,000 faculty members, clerks, janitors groundskeepers, cafetaria workers and telephone operators and part-time employment for thousands of others.

The institutions they attend will spend at least $925 million this year, the bulk of it for staff salaries, and the students themselves, many from outside the Washington area, will spend tens of millions of dollars more, a major boost to the area economy.

To make room for them to eat, sleep, study and play, the institutions they attend have spent and are spending hundreds of millions of dollars on building projects that are changing the shape of the skyline in the city and suburbs.

Pursuing goals as diverse as proficiency in scuba diving, preparation for overseas missionary work and expertise in poultry science, they will enroll in literally tens of thousands of courses ranging from advanced molecular physics to how to survive the Christmas-New Year's holidays.

This diversity is all part of their efforts to attract and keep students.

"The institutions are in a position where they want to hold on to their faculties, and they don't want to close down departments, so they have to go out and compete for students," says Richard McCarthy, associate director of the National Advisory Commission on Extension and Continuing Education.

Those students will attend schools as varied in size and mission as the students they serve. They run the gamut from the sprawling College Park campus of the University of Maryland to tiny Oblate College on Michigan Avenue NE, which has 40 students. They include three women's colleges, Trinity and Mount Vernon in Washington and Marymount in Arlington, and three predominantly black colleges, Howard University and the University of the District of Columbia in Washington and Bowie State College in Maryland.

In mission, they range from multipurpose, liberal arts educations offered by the regions's major universities to more specialized aims found at less well-known schools such as Columbia Union College and Washington Bible College, both of which focus on preparing students for religious careers.

Founded and operated by the Seventh Day Adventist Church, Columbia Union has about 900 students at its Takoma Park campus and strictly enforces church prohibitions against smoking and drinking. A vegetarian diet is served in campus dining halls and snack bars.

"Many of our kids go overseas as missionaries and about one-quarter will end up in some kind of service to the Adventist Church," says William Loveless, president of Columbia Union. "One of our major missions is training professionals and workers for the Adventist Church."

On Princess Garden Parkway in Prince George's County, Washington Bible College has ridden the evangelical movement to a record enrollment of 600 students. "Every student here majors in Bible and theology and our ultimate aim is to turn out graduates who would be involved in some form of Christian ministry," says Bob Evans, a college spokesman. "I guess we would be considered an evangelical school."

As a general rule, state-supported colleges here tend to draw most of their students from their own political jurisdictions. But in Washington, private colleges have as many students from Maryland and Virginia as they do from city, and most recruit nationally as well. In the last decade, they say, the Washington location has become a key attraction.

"This is a remarkable city," says Richard Berendzen, provost at American University. "Washington, D.C., is, in my view, one of the great but not fully recognized educational centers of the world. The value of a Washington location is becoming increasingly apparent."

"There is a vitality here that has not even crested yet," says Clarence Walton, the retiring president of Catholic University. "It's in contrast to many other cities that are a little tired."

Most educators here seem to feel the Washington location will serve as a sort of buffer against the 18-year-old drop off, but the competition for students is still keen.

To compete for students, colleges in the Washington area are increasingly moving off campus, and it is not unusual for classes to meet in church basements, community centers, office buildings or military installations. For example, every year, more than 10,000 students attend classes at the Pentagon.

George Washington University enrolls 6,500 to 7,000 students a year in 1,300 off-campus courses at such diverse locations as Lynchburg, Va., and suburban Baltimore. Last year, the program turned a profit of $1 million, bringing in revenues of $5 million against expenses of $4 million.

In the rush for adult students, universities and community colleges have added many nontraditional and innovative courses to their curriculums. Classes such as yoga and meditations, wine tasting, biorhythms, extrasensory perception and juggling have become standard fare. Catholic University even advertised a course this winter on how to get rid of your fear of snakes, while at Prince George's Community College, dietitian Carol hummerlach has taught popular courses for two years on the use of behavior modification to control weight.

Nor has the surge in adult education been restricted to the established colleges and universities. Such groups as neighborhood and community organizations, the Smithsonian Institution and the Department of Agriculture Graduate School have jumped with gusto into adult education.

Enrollment at the Graduate School soared to 29,000 this year, with students taking a range of courses from computer science to caring for plants. About 12,000 people a year enroll in courses at the Smithsonian where the menu runs from anthropology to art appreciation to a variety of studio arts.

"For many of them, it's good way of meeting people. We've become a socially acceptable alternative to the singles bar," says Janet W. Solinger, who directs the Smithsonian program.

"People have recognized that if they don't continue to learn, they're going to stand still and become obsolete. And they've reached the limit of what they can take on television."

To flesh out enrollments, schools also are turning to special groups such as women whose children are grown or in school, the elderly and the retired.

The Graduate School, in cooperation with the American Association of Retired Persons, for example, has set up an Institute for Lifelong Learning, aimed specifically at providing courses for senior citizens.

"Some people get into this for the social experience, but it also enriching," says Nancy Shields, 64, a retired English teacher who has taken course in creative writing, Spanish, drawing and Middle Eastern politics at the Institue.

"This gives me a chance to do some of the things I never had time to do when I was working."

To lure students from the ever-expanding pool of women returning to college, most schools have set up sepcial programs. One of the oldest is George Washington University's Developing New Horizons for Women, a course geared toward helping women.

Typical of the women who enroll is June Frost, self-described as the "perfect club woman . . . a good wife and mother." After completing Horizons, Frost enrolled in another course at GW designed to equip her with skills for a job in the publishing industry.

"I have reached the magic age of 40 and I need an identify separate from my husband and my children," said Frost. "I need a marketable skill."