IF I WERE a politician running for public office next year, preferably for president, I would start by making outrageous promises about the 1980s. There are good times ahead for America, and clever politicians will starttaking credit right now, before the voters know whom to thank. A clever candidate in 1980 would promise this for the next decade: A dramatically lower crime rate. The sharp decline of unemployment among young people, white and black. Significant improvements in education, with more of our children going to college and finding better jobs afterward. A big turnaround for the America family - fewer divorces, fewer illegitimate births, more marriages and more babies. A heartening decline in the social distress signals - teenage suicide and violence. In short a happy decader lies ahead, at least much happier than the Seventies have seemed.

Of course, any candidate who dwells on "good news" instead of the "big problems" will be savaged by the press. The keen-witted reporters will demand to know exactly what this politician is going to do to accomplish these improvements. At that point, the clever politician will lie. The honest politician will smile and answer: not much. Most of these happy changes will occur, with or without the government's cooperation, with Jimmy Carter in the White House or Howard Baker or Harold Stassen, even with me or thee in the White House.

The explanation is that most of these trends toward social equilibrium are inevitable products of the great population shift this nation is about to experience. Some of these "good news" trends have already started (fewer homicides, for instance), and if Carter's political wizards were more astute, he would already be claming credit for them. If politicans will restrict themselves to promising the evitable, it would do wonders for restoring the public's faith in politicans.

Right at this moment, America is at the crest of a long and fundamental cycle in the structure of its population. For the past 20 years, the yourth population grew inexorably in size, flooding schools and colleges, overwhelming the job market, pushing up the crime rate and all the associated problems of social stress. For at least the next 15 years, this will be reversed profoundly. From 1980 to 1995, the youth population (15 to 29 years old) will shrink in actual numbers, from 30.4 million to 25.8 million. Even more important, these young people of the future, most of whom are already born, will be a much smaller proportion of the working-age population. This will be good for them and good for the rest of us, too, if the government reacts wisely to the demographic trends already clearly established.

Consider the future of crime. Most violent crimes are committed by young people. They are passing through a turbulent period in their lives, a time of stress and experiment, whether they are rich or poor, unemployed or full of promise. Crime rates soared in the early Seventies as the youth population soared, notwithstanding the billions of dollars Washington gave to the police departments of America. In the 1980s, crimes of violence are going to drop precipitously because there are going to fewer young people to commit them.

But this demographic shift will yield a change far deeper than mere numbers, a structural change in age-composition of our job economy, one that should wipe out the twin aggravations of the Seventies - staggering youth unemployment, especially among unskilled blacks, and bitter disappointment among college graduates who can't find careeres worthy of their educational level. In the 1980s, young people will find themselves in a seller's market, with a scarcity of youthful laborers willing to start at the low-wage bottom of the economy.

This scarcity is going to bid up their wages, offer more choice and mobility in their careers and, thus, make it even more valuable for young people to advance their educations. Colleges willbe searching them out, desperately, as the pool of available students shrinks. Don't be surprised if predominantly white colleges and universities intensify their recruiting of black, Hispanic and other minority students, not to pursue lofty ideals of social justice but to help ease their nightmares about empty seats.

Older brothers and sisters will be grumbling about how a college degree didn't help them much when it came to finding a good job - but that experience will be reversed for kids who come to maturity in the next 15 years. The message to them should be: Stay in school, the good times are coming back for ambitious young people who want to advance their family's status in this society.

These particular changes are virtual certainties in the short run, say, over the next 10 years. The other social effects are necessarily more speculative, but demographers are working out exciting theories about what we may expect from this age shift. Again, the news is good, unfashionable as that seems.

Richard A. Easterlin, a professor at the University of Pennsylvania and past president of the Population Association of America, has developed an audacious theory on the cycle of age shifts that forecasts, among other things, a restored equilibrium for America familiies, a reversal of soaring divorce and illegitimacy rates. Easterlin focuses on the relative wage-and -employment prospects of youthful males as the central social lever; as their economic position improves in the coming years, it becomes easier for young men and women to marry and start families, with less stress, less disappointment, more real progress. Young women will benefit from this trend, too, but the effects on female wages will be less dramatic because older women will continue to come back into the job market after raising children, competing with younger women for the same jobs.

Easterlin's theory challenges the spiritual assumptions of the feminist movement. In essence, he argues that the surge of women going to work over the last 15 years is an effect, not a cause, stemming from the economic squeeze caused by that huge bulge in the youth population. Taking social measurements over a 40-year span, he predicts that younger women will opt more and more for staying home and having babies - even though the job market will be offering more and better opportunities for them.

My own hunch is that he's half-wrong. Millions of women in dreary jobs probably would choose home over work if their economic situation allowed it, but i think young women today look at their lives in a fundamentally different way than their mothers did, and their futures include careers, working for pay, regardless of economic need.

But the evidence from the past is on Easterlin's side. He traces the surges and falls in an array of social indicators, from divorce to suicide to illegitimacy, and finds that they coincide, with remarkable precision, with the growth and shrinkage in the youthful population. The Eighties and the early 1990s, based on these numbers, will be very much like the late 1940s and the 1950s - another time when there were also fewer young people in our society, when opportunities automatically expanded for all of them, men and women, white and black.

This is an intriguing insight into how America works - the generation hopscotch that goes on with vast effects, yet beyond anyone's control. Put aside all of the other labels and distinctions that divide us - political afflication, religion, economic status, race, sex. Americans, young and old, are fundamentally divided into two generational tracks - the lucky cohort, which enjoys the good times, and the unlucky one, which doesn't. We pass this identity onto our children, for better or for worse, without even knowing it.

For example, I am a member of the lucky cohort. I was born in the 1930s, when fertility rates were low, which means I went to college and looked for my first job in the 1950s, when young people were relatively scare. All through my life, I will enjoy the economic benefits of less competition from my age group. My children were born in the 1960s, when fertility was low again. They will mature in the 1980s and enjoy the same good luck that I had.

The unlucky cohort is the one in between - the children who matured in the Sixties and Seventies and experienced so much strss and disappoinment. There are too many of them. They have to scramble for limited admissions places at the best colleges and, afterward, they have to scramble for jobs and promotions. Throughout their lives, they will find the entry points for advancement clogged by too many of their peers. Coincidently, this group also seems to be the generation that fights America's wars (the deep cynics would contend that this is not a coincidence).

In any case, these unlucky Americans will pass their bad luck onto their children - who will come along in the late 1990s and discover that, like mom and dad, they are part of a demographic bulge that crimps their opportunities in life.

The most exciting elements of this demographic perspective is that it genuinely opens the door to new questions, new thinking on social change and economics. Easterlin, for instance, contends that the government's Keynesian economic policies of the 1970s failed largely because they never confronted these demographic realities in the labor market. Thus, the anomaly of high unemployment and high inflation simultaneously.

The structure of the job market was overwhelmed by new young workers. Federal programs were aimed at job training and education, when the heart of the problem was job themselves - not enough entry-level jobs to absorb the bulge. This kept unemployment rates abnormally high, but the government responded by pumping more cash into the entire national economy, which then overheated. Easterlin theories that both inflation and unemployment can be conquered in the 1980s - if government managers appreciate that they will be dealing with a fundamentally different economic structure from thatof the last 15 years.

This is the bad news for the feds: a very tricky budget situation, utterly unlike the past. As the population ages, the heavier shift from the state and local governments - which pay most of the costs for education and police protection and other servicesassociated with young people - to the federal treasury, which pays most for maintaining the elderly, sick and disabled. We are going to see a decade in which mayors and governors grandstand on the "balanced budget" issue while Washington finds it more and more painful. The first victim will probably be that "free money" called revenue-sharing which the feds now send out to cities and states - a cruel joke on the real priorities.

In the future federal managers must find ways to maintain healthy economic growth, despite all the natural limits, but the political aggravations caused by stagnation are going to move upward in age brackets as the "baby boom" children reach middle age. At 40, millions of them are going to be really mad. Meanwhile, consumer demand will be more difficult to sustain because the formation of new households - young families buying cars, appliances, homes - will slow down in middle Eighties. A new ball game awaits the economic wizardss; let's hope they do better than they did with the old one.

The next decade, nevertheless, could be extraordinarily progressive time if the American people, not to mention the politicians, will grasp the possibilities. Government programs to help young people advance their economic status could be relatively more modest in size, yet also dramatically more effective than they have seemed to be in the past. There will be fewer young people who needed the extra help, and the opportunities for them will be naturally brighter. Rearranging social equities, among the well-off and the poor, among the white and black and Hispanic, can be accomplished in those circumstances, with less pain, even with less money, if the government is more sophisticated about where to spend its dollars.

First, the nation has to get over the gloom, from the past, the conventional wisdom that nothing works. The demographic cycles described by Easterlin and others suggest to me that a major re-evaluation ought to be undertaken of the Great Society's alleged failures, one that takes account of the burgeoning age-shift which confronted those social programs.Il suspect, if academics began making those comparisons, they would find that the Great Society youth programs did not fail in relative terms - they were overwhelmed ny the unlucky cohort.