AS THE POST-WAR "boom" babies have come of age, American education have faced with what can be described as an "empty-desk syndrome." The steady enrollment decline now besetting public schools will next affect colleges and universities.The number of 18-year-olds is expected to fall 21 percent within the next decade. To stay live and healthy, schools and colleges are energetically developing a new market: the adult student. And they are doing so with measurable success.

At Georgetown University, the 7,000 adults enrolled in the school of continuing education now outnumber regular undergraduates. At George Madson University, more than 60 percent of the enrollees are part-time students. Some colleges provide special serivices which help attract adult students.All three Montgomery College campuses incorporate day-care centers, and Northern Virginia Community College offers an intensive weekend lecture program called "Weekend College Course a Month" specially aimed at adults with full-time jobs who have no time for courses during the week.

Local school districts are also finding that adult education offers possibilities for growth, and many in the area offer courses in subjects ranging from antiques to remedial arithmetic. Frank Snyder, director of adult education for Montgomery County, says, "We're always expanding. You have to keep changing your program because people's interests are changing."

Many of the adults in continuing eduction courses are women -- 63 percent in Montgomery College's non-credit courses. They enroll -- whatever their sex -- to sharpen or learn skills for better job opportunities or at work toward a delayed bachelor degree or high-school diploma. Some are filling leisure time with courses they consider worthwhile and enriching.Almost all appear to be in the classroom by choice, and to value their opportunity to learn.

NEED A GOOD old-fashion bleeding? Want a tooth pulled with only a shot of rum for pain? Call John Victor.

At age 59 Victor will graduate this spring from George Mason University where he is majoring in 18th-century medicine as part of the bachelor of individual studies, a degree program which lets the student develop his own curriculum.

After his retirement from the National Security Agency, Victor took an unwitting step toward his return to the campus. In 1975 he helped to organize and then became cammander of the 1st Virginia Regiment of the Continental Army, a group set up to help-celebrate the Bicentennial.

"At one of our parades, I met a woman who had medicinal herbs and powders of the period. It fascinated me, and I decided right then that I didn't want to be commander anymore. I wanted to be the regimental surgeon."

Victor's Walter Mitty-like interest in old-fashioned medical arts led him to a little independent research, and he eventually became one of 2,750 students in George Mason's continuing education program. In light of his age, Victor says he is known as "Dad" around campus.

He outlined a curriculum with an adviser, and began his studies.

"I feel better now than ever," says Victor, who recalls hating his years at NSA. "I hated to get up in the morning, to go to work and even coming home. Now I'm out of the stagnant world and into a very stimulating environment."

As part of degree requirements, Victor has been speaking publicly about his studies, has assembled a display of 18th-century medical instruments and powders and has fashioned an outfit that a doctor of the period might have worn.

He wants to become a curator of an apothecary or medical museum.

"This has been like an adventure and now the practice is almost over. I've got an opportunity to do something. I know what I want and I've got something I didn't have before -- enthusiasm."