Rosalynn Carter, who a month ago launched a program to focus attention on the nation's mentally disabled, yesterday began an effort to help with the problems of the nation's 22 million elderly.

"A society that cares is the most important thing," the President's wife said yesterday, concluding the first of what she sees as a series of White House round-table discussions on problems of the aging. "I know if we work together we can do something worthwhile," she said.

Using much the same procedure she did a month ago with members of the President's Committee on Mental Retardation, she sought recommendations from 23 experts on aging on such questions as mandatory retirement, health care, housing and nutritional needs.

She not only got them - in five-minutes bursts, carefully clocked by an aide seated behind her - but some others she may not have anticipated.

"There is a need at the highest levels of government for a prominent, national advocate of the elderly to stimulate greater national awareness of their problems," said Bert Seldman, director of the AFL-CIO's Department of Social Security.

He proposed that she expand upon the task she had undertaken by setting up the meeting and that she become, in effect, "a national ombudsman" for the 22 million Americans over 65.

Rep. Claude Pepper (D-Fla.), chairman of the House Select Committee on Aging, saw the advocacy role from another perspective.

"For too long the elderly have suffered the indignity of a confused and confusing bureaucracy, the indignity of mandatory retirement, the indignity of a system which favors costly and often needless institutionalization, the indignity of nursing homes which are too often tinderboxes and the indignity of stereotypes which distate that to be old is to be feeble-minded and worthless," said Pepper, at 76 teh senior among his fellow conferees and therefore first to commence the discussions.

He invited the First Lady to put pressure on his congressional colleagues "to bring legislation (benefiting the elderly) to the floor out of committee where it reclines."

"You are suggesting that I lobby the Congress?" She asked Pepper, who had told her that as committee chairman he would "welcome" her counsel. "I'm willing to do that. I do understand how important the programs are," she said.

Later, the First Lady expressed satisfaction that the discussion had "been a start," although she said there had been no way to invite everyone concerned with topics under discussion.

"From this I'll get a focus on what to do," she said, adding that she had no time limit because "I've got four years . . . I wanted to start now, but the goals are long-range."