The American work force is changing as dramatically as the American work place.

Within the next two decades, the 104 million people who do the jobs in this nation will, on the average, be older and better educated than those working today, and the proportions of females and blacks will be greater than they are now.

Workers will be less likely to be members of labor unions, more likely to live in rural areas and more likely to speak a language other than English at home.

These changes involve not small percentages, but tens of millions of people. Before the end of the century, they are likely to cause sweeping changes in work and business, affecting product development and marketing, the provision of services, education and training, and pension and health-care policy.

"Demographically, these are the most dramatic times in our history," says George J. Stolnitz, professor of economics at Indiana University, in a paper to be published by "Business Horizons."

"They are also, almost surely, the most unpredictable and problematic. Major trends which can be traced back to our earliest records have, with small warning, reversed course or become altered beyond recognition, while others which have persisted have, paradoxically by that very fact, brought us to uncharted and clearly choppy waters."

Demographic analysts and labor experts agree that continued rapid growth in the number of working women will probably be the most significant change in the work force for the rest of the century. But they disagree on whether women by their very indispensability will achieve gains in salary and status, or will be driven by their very numbers into competition with each other, to their collective detriment.

Half of all American women of working age are already in the labor force, which means working or actively looking for work. Within the decade, the figure is expected to rise to at least 61 percent, and women will make up most of the growth in the overall labor force--a phenomenon with vast implications for work-place competition, demands for day care and flexible hours, and retirement benefits.

At the same time, the number of teen-agers and young people entering the labor force is plummeting. The "baby boom" generation now passing into maturity had far fewer children than their parents, and the number of people aged 16 to 24 will actually decline in the 1980s.

This will force employers who rely heavily on youth, such as the fast-food industry and the armed forces, to alter their mix of workers. It will force the producers of trend-dependent products, such as clothing and records, to change their styles. And it may, according to demographers, dramatically reduce the nationwide problem of youth unemployment--but probably not for blacks, whose ranks are not decreasing like those of whites.

According to the National Commission for Employment Policy, the "youth share" of the labor force is expected to decline from 23.5 percent to 18.4 percent during the decade of the 1980s.

This, the commission says, "should make it easier for the current generation of young people to find jobs and prepare for adult work roles. It is not clear, however, whether the changing demographics will be of much benefit to the youths who have been most at risk. In particular, the numbers of black and Hispanic youths in the population and labor force are not expected to decline significantly during this period."

These dramatic changes are occurring simultaneously with a new revolution in the national economy that is demanding higher skills and special abilities in a high-technology work place. They also parallel other dramatic shifts in the population and work force whose implications are only dimly perceived:

* The population of rural counties, after decades of decline, grew in the 1970s at a rate equaled only in the 1850s. At least 3.5 million Americans moved out of urban and suburban communities into "nonmetropolitan areas" in the 1970s, and rural population experts expect the trend to continue. Interstate highways, electronic communications, the community college system, low land costs and a paucity of unions are making rural areas attractive to industry, and factory work is supplanting farming as the employment of much of the rural population.

* The trend toward early retirement among male workers slowed sharply in the 1970s, and some experts believe a new trend toward retiring later is about to develop. With inflation undercutting pensions and shortfalls in the Social Security fund causing doubts about retirement security, more workers are expected to stay on the job longer--especially since fewer young people are coming into the work force.

* In between, in the prime working years, are the children of the post-war baby boom, known to demographers as the "pig in the python" because they represent a vast, distorting bulge in the population as they pass through life. So great are their numbers that demographers and labor experts foresee difficult years for them as they compete with each other for salaries, promotions, job status and work satisfaction in their prime earning years.

In 1975, according to the Bureau of Labor Statistics, there were 16.7 million men and women in the work force aged 35 to 44. By 1995, BLS projects that the number will be at least 33.7 million.

"In this decade these folks will be competing for promotion and advancement not only with themselves but with their fathers and uncles who are deferring retirement," said Jerome M. Rosow, president of the Work in America Institute.

Unlike the preceding generations, who were children of immigrants and survivors of the Great Depression, the workers of the postwar era have not rushed into labor unions.

At the end of World War II, more than a third of all nonfarm workers were members of unions. By 1978, according to Martha Riche, associate editor of American Demographics magazine, the ratio had dropped to less than a fourth.

According to Riche and other analysts, current labor trends do not favor unions. Decentralization of industry, an aging and better-educated population, an increase in part-time employment and the proliferation of small businesses--the primary source of new jobs--combined to discourage union organization.

The unions, according to Riche, failed to respond to the great growth of female employment, partly because the sectors of the economy where unions are strongest, such as manufacturing, mining and transportation, are those most dominated by male workers.

Demographers say that work-force trends can be predicted with accuracy because everybody who will be working in the 1990s has already been born. But they acknowledge that their projections omit the one great imponderable that will affect labor and industry in the coming decades, immigration. About 400,000 immigrants enter the United States legally each year. But there are also an estimated 3.5 to 6 million illegal immigrants, and their ranks are growing by an estimated 400,000 to 500,000 a year.

The current rate of legal immigration alone could add 12 million persons to the population by the end of the century, demographers say. The ranks of illegal immigrants and refugees are likely to swell the work force still further, though the overall impact that would have on the job market is unclear.

Charles B. Keely, immigration and refugee policy analyst at the Population Council in New York, said the unanswered questions about the flow of aliens into the country include these:

What will be U.S. policy on admitting refugees as political upheavals continue in Asia, Africa and the Caribbean?

What will be the impact on industry if Congress enacts pending legislation to revamp the immigration laws and jail employers of illegal aliens?

Could the economy continue to function if all the illegal aliens were deported?

"Are they really deportable?" he asked. "Would Americans do those jobs?"

Some industries, such as agriculture, food services and garment manufacturing, are virtually dependent on illegal immigrant labor, he said, and without it either would have to find other sources of low-cost labor or raise wages and improve working conditions.

Some industries have begun to start thinking about the implications of the changes in the work force.

William Daly, former vice president for planning at Amtrak, for example, said the railroads are contemplating a future in which "there will not be a supply of workers who want to spend most of their adult lives away from their families." The railroads, he said, "will have to develop technology that will minimize the need for workers to stay away--that is, nobody up in front."

Keith Wardell, a consultant to the convenience-store industry, said stores such as 7-Eleven "are seeing a very dramatic change in their customer profile." More and more customers, he said, are "working females and those over 50," who have more money and shop less often than the young, single customers who predominated in the past.

These new customers demand a more sophisticated mix of products, he said, and that in turn requires a more mature, better-trained staff. "That drives up labor costs," he said, "which means you have to sell more higher-priced items. It's a spiral."

Many demographic specialists, however, say that industrial planners have only begun serious scrutiny of the trends, and in particular of the most dramatic change of all, the growth of female employment.

Not only are more women working, they are staying in the labor force longer and bringing higher qualifications with them.

Women still earn on the average only 60 cents for every dollar earned by men--a relative value first dictated by God to Moses in the Book of Leviticus--but some analysts believe their increased education and "on the job training" will move them toward equality.

Women may find greater opportunities in new industries such as computers and electronics where physical endurance is not required and traditional patterns of male dominance are not so entrenched.

But other experts say that women have failed to move out of jobs they traditionally have held; sexual integration of the work place is occurring more as a result of the movement of males into such occupations as nurse and telephone operator than the other way around. Thus women may fall further behind because of greater competition.

In addition, many working women have young children, and may find opportunities for advancement limited by lack of day-care facilities.

"Many women with young children," according to the Population Resource Center, "cannot conform to traditional job structures, creating the necessity for flexible work patterns and day-care facilities. The current demand for child care is 16 billion hours per year, yet there are fewer facilities now than there were in 1945."

Since more women are in the work force on a long-term basis, the current system of pension and health benefits in many firms is obsolete because it is based on vanished patterns of family life, experts say.

Stephen Sandell, an analyst at the National Commission for Employment Policy, predicts that by 2000 as many as 80 percent of women reaching the age of 65 will qualify for retirement benefits on the basis of their own earnings. Since women live longer than men, pension funds may have inadequate resources.

The Employe Benefits Research Institute, reporting the results of a forum on these issues, said that "conventional benefit plans and entitlement programs are no longer efficient" in an economy increasingly dominated by single and divorced persons and working women.