Social Security, with outlays estimated at $181 billion in 1984 for more than 36 million people on the old-age and disability rolls, is by far the largest source of income for the nation's aged.

In 1982, there were 59 percent of the aged who received at least half their cash income from Social Security, according to a recent study by the Department of Health and Human Services, which administers the program.

The same study estimated that in 1980, of all cash income received by all the elderly, Social Security payments made up 39 percent. The next largest source was income from assets, 22 percent, followed by earnings from work, 19 percent. Private pensions contributed 7 percent.

The maximum benefit for a worker retiring at 65 in 1985 will be $725, and if that person has an aged dependent husband or wife, another 50 percent could be received as a "spouse's benefit" for a total of $1,087 a month.

Average benefits, however, are lower. The average aged couple this year gets $750 a month including the "spouse's benefit," which will rise on Jan. 3 to $776 as a result of the 3.5 percent automatic cost-of-living increase announced in October.

The maximum tax rate for an employe will rise in 1985 from the present 6.7 percent on the first $37,800 of wages to 7.05 percent on the first $39,600, which means that workers at the maximum will be paying $2,791.80 in 1985 compared with $2,532.60 in 1984. Similarly, the tax for the self-employed will rise from a net rate of 11.3 percent to 11.8 percent on the first $39,600, which means a self-employed person at the maximum would pay $4,672.80.

In contrast to 1983, when the old-age and disability system faced imminent bankruptcy until Congress and the president agreed on a rescue package, 1984 saw no financial crisis.

The system's trustees announced in the annual report that barring a major recession in the next few years, the trust funds (excluding Medicare) are solidly financed as a result of the 1983 legislation and will be able to pay all benefits until about the year 2060 except under the most pessimistic set of economic and demographic assumptions; and even under that pessimistic scenario the old-age and disability system is safe until around the year 2020.

For Medicare, which is legally a part of Social Security, the outlook is quite different. Despite recent cuts in Medicare, which actuaries estimate will help produce one added year of leeway, the Medicare hospital insurance trust fund, which is financed from part of the Social Security payroll tax, is expected to go broke permanently between 1990 and 1995.

And while the second Medicare trust fund -- the doctor insurance trust fund -- will not technically go broke because it automatically draws three-quarters of the money it needs directly from Treasury general revenues, its costs also are rising rapidly. The reason for these gloomy prospects is the general rate of medical inflation in the economy which in most years has been about double that of general inflation.

While many on Capitol Hill are talking of a Medicare rescue package in 1985 -- possibly consisting of higher taxes, further squeezes on per-case and per-visit payments to hospitals and doctors, benefit cuts and/or increases in out-of-pocket charges to beneficiaries -- it is unclear what will occur.

The major Social Security legislation in 1984 was a bill revising disability insurance program rules to make it harder for the government to remove people from the rolls on grounds they are well enough to work.

Reviews that had started in 1981, following a 1980 congressional mandate, produced so many "horror stories" of severely disabled people being thrown off the rolls, so many accusations against the Reagan administration for alleged excess zeal in carrying out the reviews, that Congress and the White House, which had initially resisted easing of the law, finally agreed on a legislative compromise.

Its main feature was a requirement that henceforth, medical improvement in an individual's condition must be shown before he or she can be removed from the rolls.

While the government projects that the combined old-age and disability trust funds are in sound condition for many years to come, there is a school of thought, led by men such as former Social Security chief actuary Haeworth Robertson, which holds that the assumptions used for future projections may be too optimistic.

They argue that severe future financing problems are masked by the fact that current projections only go out 75 years into the future and therefore do not capture the full impact of the aging of the population in the next century, when the post-World War II baby boom generation retires and the proportion of the population 65 and over will rise from the present 11 percent to nearly double that.

With regard to the assumptions, Robertson and others have argued that the so-called intermediate scenario, the one usually considered to be the most likely to occur and which is used as the basis for policy decisions, assumes unemployment will permanently level off at 6 percent annually and inflation at 4 percent. If things are only sightly worse -- one percentage point in each case, accompanied by certain other changes -- the Social Security annual report shows there will massive trust fund deficits instead of a virtual balance over 75 years.

Some analysts believe that if these warnings are valid, and there is substantial dispute about that, a solution might be to gradually cut back benefits to a level that would be sustainable from the current payroll tax even if the worst economic scenario came about.

Others, like former White House aide Peter J. Ferrara, have proposed plans to allow workers to invest the money they ordinarily would pay into Social Security into private trust accounts, similar to Individual Retirement Accounts, dedicated to producing retirement income and to paying for medical benefits on retirement. Part of the funds would also go into private insurance to meet various contingencies for which the existing Social Security system provides protection.

Ferrara contends that these IRA-type instruments would provide more private capital to stimulate the economy, and ultimately, as a result produce far more returns than Social Security can pay on the amount put in over a lifetime.

These schemes appear so far to have gained few adherents. Many economists strongly doubt these proposals would work out as advocates claim. Democratic politicians, such as Walter F. Mondale, solidly oppose them. President Reagan during the campaign made a nearly absolute pledge that he would not tamper with the basic Social Security system nor cut old-age and disability pensions either for current or future beneficiaries.

Perhaps a greater possibility of a change in the benefit structure over the next few years arises from the massive size of the federal deficit. Social Security and Medicare are the two largest domestic programs, and while the money in the trust funds cannot be used directly for other purposes, reductions in program outlays would shrink the ostensible size of the overall combined federal deficit.

It is conceivable that if deficits become the overwhelming domestic issue, Congress may attempt, despite today's promises and tremendous opposition from organizations of the aged, to cut Social Security as well as Medicare benefits. One way suggested by budget-cutters in the past is to hold the annual automatic cost-of-living increase in Social Security benefits to less than the actual increase in the Consumer Price Index over the preceding year. For example, if the CPI in a given year rose 5 percent, the cost-of-living increase in Social Security benefits at the start of the next year might be held to 3 percent.