Among the younger members of today's work force, it is fashionable to be cynical about Social Security.

"I'll never see a dime of what I put in" and "I'm not counting on it to do me any good when I retire" are the kinds of sentiments often heard at yuppie cocktail parties.

That attitude has been brought about in part because twice in the past 10 years Congress has had to rescue the system's main trust fund from insolvency and in part by the unfavorable demographic pattern confronting the system: a rapidly growing number of retirees drawing benefits while a much smaller number of workers pay into it.

But the people who run Social Security say that they believe the problems are controllable and that the system will be there, paying real benefits, when today's workers retire.

It's hard to imagine the country without it. Social Security today and for the foreseeable future is the single largest source of income for the aged, supplying about 38 percent of cash income for people 65 and over, compared with 28 percent from assets, 16 percent from jobs, 7 percent from government-employe pensions and 6 percent from private pensions.

Social Security provides more than half the support of the low-income aged, since much of the income of the elderly from assets and earnings is received by a small minority. Thus, about three-fifths of the elderly depend on Social Security for at least half their income, and for many it is virtually the only source of income. It is a universal system and nearly all the retired elderly are eligible for benefits.

The program is huge, second only to defense among federal programs. About 39 million people -- including 4 million people on Social Security disability -- will draw monthly benefits in 1988.

Projected 1988 costs are about $222 billion for old-age, survivor and disability insurance -- OASDI -- benefits. Medicare, Social Security's "baby brother," which goes to Social Security recipients who are at least 65 or are disabled, will have outlays of about $85 billion.

It is expected that private pensions will play a larger role in the income of the elderly as people who entered pension systems during their growth heyday in the post-World War II era begin retiring in ever larger numbers.

That is as President Franklin D. Roosevelt and others who created the system in 1935 intended. Social Security was never intended as a total support in retirement, but only as a base to supply about two-fifths to one-half of the amount needed for retirement -- a base on which an individual could build through earning private pensions on the job and through private savings.

The system is financed by a payroll tax paid by virtually all workers -- currently 7.15 percent that employes and employers each pay on the first $43,800 of the worker's salary. The rates in 1988 will be 7.51 percent each on the first $45,000. The maximum taxable wage rises every year to keep pace with the rise of wages in the economy, but the tax rate as of now is scheduled to reach a maximum of 7.65 percent each on employers and employes in 1990. No further increases are scheduled.

The Social Security Administration recently announced that the annual cost-of-living adjustment to keep benefits from lagging behind inflation will be 4.2 percent for 1988, starting with checks received Dec. 31 of this year because the normal payment date of Jan. 3 falls on a Sunday.

As a result of this increase, the average benefit for a single retiree in 1988 is projected at $513 a month; for a couple it is projected at $876. The maximum benefit for an individual retiring in 1988 at age 65, who has worked steadily at high wages most of his or her life, will be $838 a month.

The Dec. 31 payment date brought some tremors from some individuals who reasoned that it would boost their income taxes, since it would mean they received 13 payments in 1987. Up to half the Social Security benefit is subject to income tax if the individual had countable income exceeding $25,000 a year, or $32,000 in the case of a married couple.

However, former Social Security chief actuary Robert J. Myers points out that a special provision of law classifies the Dec. 31 payment as income for 1988, not 1987.

Although most people think of Social Security as primarily a retirement program, it is actually much more, providing income-insurance throughout a person's working life against the hazards of disability and death.

In addition to getting retirement benefits that can start as early as age 62, a younger worker covered by the system, even one in his or her 20s, is guaranteed disability benefits (including dependents' benefits) if found to be so disabled as to be unable to work at any job. In addition, survivor benefits are payable to a spouse with small children if the primary wage-earner dies.

Of the 7.15 percent tax, only 5.2 percentage points goes to support the retirement system and survivor-benefits system. Another 0.5 percentage point goes for the disability trust fund, and the remaining 1.45 percentage points for the Medicare hospital trust fund.

Based on these numbers, the scheduled tax increases, and the best current estimates on income and benefit payments, Social Security Commissioner Dorcas R. Hardy answers an unequivocal "yes" to those who ask if the system will survive. Social Security will be able to pay all benefits, she says, "well into the next century."

These estimates are based on a standard scenario projecting economic conditions, birth rates and other factors 75 years into the future.

According to that estimate, Social Security will build up a giant reserve in its OASDI trust funds as long as most baby boomers are still working, equal to about $2.5 trillion in 1987 dollars by the year 2020. As they retire, leaving a thinner generation behind to pay the Social Security taxes that support the system, the trust funds will decline rapidly, becoming insolvent between 2050 and 2055.

But at that point, an increase of about 1.4 percentage points each in the 7.65 percent payroll tax on employers and employes would maintain solvency into the indefinite future.

However, some students of the system, including former chief actuary Haeworth Robertson, have repeatedly criticized the official projections as too optimistic. The standard "intermediate" scenario on which the projections are based assumes economic growth of 2 to 3 percent a year until around the year 2000, inflation leveling off at 4 percent, and average unemployment leveling off at 6 percent.

If conditions were only a bit worse (the so-called pessimistic scenario, with slower growth, 5 percent inflation and 7 percent unemployment), the combined OASDI trust funds would go broke before 2030 and would need an increase rising to about 6 to 7 percentage points by 2050 each on employers and employes, not 1.4 percentage points, to keep solvent.

For the moment, there is little concern on Capitol Hill about what could happen 40 to 50 years from now. A projection based on the consensus intermediate estimates that predicts solvency until about 2050 looks pretty good to most members.

But if the economy were to lag over the next generation and fall substantially below the intermediate projections, there would undoubtedly be an upsurge of concern about the long-range solvency of the system, with appropriate action likely to be taken to ensure i