Older Americans have probably never had it so good, yet they are not secure. They are extremely concerned that they will lose what they have.

By their own estimates, most have been freed of many burdens that beset earlier generations of older people. Adequate housing, the cost of medical care, and getting around are no longer serious problems for the great majority of elderly citizens today.

At the same time, according to a new Washington Post-ABC News poll, older citizens fear that the relative comfort they have become accustomed to is in jeopardy. Many believe the government will cut their Social Security benefits, and most are persuaded that the private sector--meaning their children, in this instance--won't pick up the slack.

A plurality of older people also say, notwithstanding their general sense of well-being, that not even current levels of government aid are adequate. Young and middle-aged Americans feel to an even greater extent that the government is not doing all it should for the elderly.

If one theme more than others jumps out from the new poll, it is the central role that Social Security plays in the lives of the nation's elderly.

Some 23 million people collect Social Security retirement benefits. For 57 percent the monthly payment, ranging from about $170 to $750, represents more than half their household income, according to the poll. For 40 percent, Social Security equals three-quarters of their income or more.

And while many senior citizens continue to live in or on the edge of poverty, these Social Security benefits--indexed to keep up with inflation--plainly help.

Judging by their responses to the poll, older Americans appear to have no more difficulty in meeting their expenses than do younger ones, whose incomes often have not kept pace with inflation in the last few years. Only 10 percent of people aged 62 and older, for example, say that adequate housing is a serious problem for them. But among those under the age of 62, 18 percent say housing is a serious problem.

Among older people, 19 percent told Post-ABC News interviewers that not having enough medical care is a serious problem for them, compared to 23 percent among the rest of those surveyed. Three in four older citizens answered affirmatively when asked if they had "enough medical benefits" to get the care they need. That is about the same percentage as for the rest of the public.

Of those surveyed who are 65 or older, 43 percent reported annual household incomes under $8,000. Among the rest, only 12 percent reported incomes that low. Those figures, according to Harold Sheppard, associate director for research of the National Council on the Aging, show he "fantastic importance of Social Security in the lives of these people."

But many believe the Social Security program is about to be sharply curtailed.

Despite statements by President Reagan that he will not seek to reduce benefits for people now receiving Social Security, a majority of those interviewed, 51 percent, believe that he will. Typical was this comment from a Long Island woman who collects Social Security payments of less than $250 a month and does part-time babysitting and sewing to help make ends meet:

"He wants it to go back to the way it was when he grew up, where the young people helped out the old," she said. A 70-year-old Republican, this woman voted for Reagan in 1980 but says she would switch to independent candidate John B. Anderson if she had it to do over.

In all, 41 percent feel Reagan will not seek to cut benefits. Some among them think the president wants to make cuts but cannot afford to; "it would be political suicide for him," said a 68-year-old Chicago man, a retired Navy Department worker.

The president's approval rating dropped sharply among the elderly last May after he first proposed large long-term Social Security cuts, a proposal he later withdrew. Among those in the new poll who get most of their income from Social Security, a predominantly Democratic group, a majority said they voted for Reagan in 1980. If the election were held now, 44 percent said they would vote for Jimmy Carter and 32 percent for Reagan.

Reagan aside, a strong majority of 57 percent feels that, in five years,, the Social Security program will no longer "provide a significant share of living expenses for the elderly." Only 36 percent feel that it will.

For the masses of Americans, future cuts in Social Security would be only one more mistake in this nation's handling of the elderly. Most citizens feel the government is not doing nearly enough right now, even though more than a fourth of the federal budget goes to the elderly.

One question in the Post-ABC News poll was: "How about the U.S. government: Given all the expenses the government has, would you say it is spending as much as it should on the elderly, more than it should, or not as much as it should?"

Two in three say the government is not doing as much as it should. Strikingly, among those under the age of 62, more than three in every four take that view.

An overwhelming majority--84 percent--said that "under no circumstances" should Medicare, the federally supported medical program for the elderly, be cut back despite cuts in other programs being dictated by economic woes. Similarly, a majority said they did not think that government should cut spending for social programs in order to reduce the expected deficits in the federal budgets.

As the Long Island woman intimated, Reagan has expressed the belief that ordinary Americans will contribute more as government social programs are cut. Citizens tend to accept that view in the abstract, with 58 percent in the survey agreeing that "most Americans will make major sacrifices to help their elderly parents," if need be.

However, they are more skeptical about situations closer to home: 46 percent say that in their own families children do not give their elderly parents as much financial assistance as they should. Only 35 percent say children in their families are helping their parents enough.

This one poll, of course, cannot demonstrate that older people today are more concerned about the future than their parents were, or their parents before them. But one piece of evidence suggests how attitudes of confidence have been waning.

One question in the survey asked whether people feel they are better off or not than their parents were at the same age. Among older citizens, 72 percent said they were, a figure slightly higher than for the rest of the population.

But when asked whether their own children will be better off when the time comes, 44 percent of the elderly said yes