As teen-age population declines, demand for elderly workers is expected to grow

''I went by two or three times and thought, 'They'll never hire me. Who wants a 69-year-old woman?' '' That was three years ago. Today, Caroline Grasso works six days a week, part-time, scouring tables, stocking the salad bar, and orchestrating birthday parties at the Papa Gino's restaurant here. Her name is practically a fixture on the Employee-of-the-Month plaque hanging in the lobby.

But at almost 72 years of age, Mrs. Grasso is an exception in the fast-food business.

''We would give anything to have 100 like her,'' says Rick Whitman, personnel director of the seven-state, Italian fast-food chain based in Dedham, Mass.

In coming years, Mr. Whitman and other personnel directors around the nation may be getting the opportunity to hire more Mrs. Grassos. The baby boom is busting. For restaurants, banks, insurance companies, grocery and department stores, and hospitals, this will mean fewer teen-agers and college graduates to draw on. Population trends show that the mid-Atlantic and North-Central regions will be most affected. And the shortage may be especially acute in New England.

In the 1970s the labor force of 15- to 29-year-olds grew at a brisk 19.9 percent in New England, and 26.7 percent in the United States as a whole. But that same age group will decline at a rate of 9.6 percent through the 1980s and 15 percent in the 1990s in New England. Comparatively, a nationwide drop of 6 percent is expected in the '80s and 2.5 percent in the '90s. This is according to a recent paper by George S. Masnick at the Center for Population Studies at Harvard University.

For two years now, the teen-age population has been shrinking. But businesses have not yet been touched. In fact, unemployment among youths remains high. During the latest recession, many laid-off workers and mothers flooded into these entry-level jobs. But labor experts predict that as the economy recovers and the teen population continues to fall, companies such as Papa Gino's will be hanging the ''Help Wanted'' sign and raising wages.

Since many of the industries likely to be affected are not yet feeling the pinch, most are just monitoring the situation. But the restaurant industry is particularly aware of the potential problem. It is highly dependent on teen labor and the industry's rapid growth may compound their difficulties.

''The fast-food work force will jump 51 percent by 1990,'' says Dorthy Dee of the National Restaurant Association (NRA).

While Merrill Lynch's fast-food analyst, William Trainer, doesn't see the diminishing teen labor supply as ''a serious roadblock to growth,'' he does predict that ''you're going to see more imaginative ways to staff restaurants.''

Indeed, the NRA is polishing the industry's image, in hopes that it will become more attractive to job-seeking teens. ''We're not the industry of first choice,'' says Ernie Royal, a member of the NRA's Human Resources Committee and owner of the Royal Hearthside restaurant in Rutland, Vt. ''Guidance counselors tend to push other careers. We're working with educators so that students know of the opportunities in the industry.''

Many companies, including McDonald's and Burger King, already use an adopt-a-school program to give students work experience and give the companies a head start in siphoning off prospects from the shrinking labor pool.

Rick Whitman at Papa Gino's believes that such efforts will meet with limited success.

''We used to go to college recruitment fairs but found they were unproductive. The job (of store manager) is too physical and requires long hours.'' To broaden the appeal, he says, at Papa Gino's they are starting to shorten the hours and reduce the physical labor involved.

George Masnick at Harvard has suggested that minority teen-agers, traditionally subject to high unemployment, might benefit from the decline in teen laborers. But others disagree. In the competition for part-time jobs, mothers and the elderly have an educational edge.

''The lack of basic skills (reading, writing, and math) among minorities is a great disadvantage. I've talked with fast-food managers and it's a very hard call to make, but given a choice between a mother and a minority teen-ager - frequently it's the teen-ager that will lose,'' says Andrew Hahn, an economist at Brandeis University.

Beyond the educational advantage, there are other assets mothers and older workers bring to the job.

''We found that our part-time older employees are more reliable - have a better attendance record, and are more willing to work flexible hours,'' said Robert S. Lee at a recent conference in Boston on older-worker employment. Mr. Lee is president of Hotwatt, a small manufacturing firm in Danvers, Mass.

Job turnover, a major drawback with employing teen-agers, is much lower with older workers. And, companies say, employees such as Mrs. Grasso at Papa Gino's, are an important role model for younger workers.

A number of firms have discovered these advantages, including Travelers Insurance Company of Hartford, Conn., Bankers Life & Casualty, Chicago, Ill., and Polaroid Corporation, Cambridge, Mass. In Florida, too, with the large retirement population it is not unusual to see seniors bagging groceries at supermarkets or flipping hamburgers at the local fast-food outlet.

Nonetheless, labor experts say that throughout the country there remains considerable attitudinal and policy hurdles to leap before older workers are accepted as a youthful alternative.

Earlier this month, support for such changes in Massachusetts came in the form of a check - $1 million from the federal government. The state is using funds provided through the Job Training Partnership Act to sponsor innovative ways to open the job market to disadvantaged women, and laid-off or low-income older workers.

''We're looking for truly original ideas,'' says Dr. Peter Doeringer, director of the Institute for Employment Policy at Boston University and member of the Massachusetts Task Force on Older Workers. When pressed for an example, Mr. Doeringer offered: ''How about using school buses to drop older workers off at the local Pizza Chef after delivering kids to school in the morning.''

Mrs. Grasso at Papa Gino's, says the idea has merit. When asked what kept friends her age from working, she cited lack of transportation, and fear. ''It's too bad more women don't get involved. I would think they would want to get out of their closet,'' she says.

The example set by this hard-working aproned hostess has a great deal to do with a hiring memo Rick Whitman plans to send to Papa Gino's store supervisors. ''I want them to seriously consider anyone over 40 that applies,'' says the personnel director.