When President Reagan stepped before the cameras and microphones in the Rose Garden on April 2 to declare May "Older Americans Month," his comments were cheered by opponents of age discrimination.

"I will back legislation which eliminates mandatory retirement requirements in government and private industry based solely on age," the 71-year-old Reagan said. "When it comes to retirement, the criterion should be fitness for work, not year of birth."

In the following days, administration officials told key members of Congress that they would support a simple change in existing law, to ban any kind of job discrimination against most workers over 70.

Now, however, after a concerted lobbying effort by business interests that want to weaken the law, the administration appears to be backing away from its original commitment.

"The president has switched signals on the Congress and the American people," charged Rep. Claude Pepper (D-Fla.), chairman of the House Select Committee on Aging. "President Reagan seemed to be making a public commitment in support of extending job rights to Americans over 70, but now apparently he is willing to go only part way or, worse yet, to walk away from that commitment altogether."

The issue is not merely partisan. Pepper's Senate counterpart, John Heinz (R-Pa.), chairman of the Senate Special Committee on Aging, also is disturbed by the administration's apparent shift. "There seems to be a misunderstanding somewhere in the bureaucracy that has to date blocked the administration from cooperating with those of us in Congress who wish to proceed with this simple bill," Heinz wrote Reagan last Thursday.

Although administration officials insist they have not changed their stance, the White House has been thrust into a new dispute with the nation's elderly groups, which already are fearful of possible budget cutbacks in Social Security and Medicare.

The Age Discrimination in Employment Act, with a few carefully stated exceptions, makes it illegal for a business to fire, demote or refuse to hire an employe between the ages of 40 and 70 solely on the basis of age. After Reagan's statement, one administration official confirmed telling congressional staff members that the president supported a simple removal of the upper age limit, leaving the rest of the law's protection intact.

In recent weeks, however, groups such as the Chamber of Commerce and the Business Roundtable have raised objections to this approach, saying it would leave their members vulnerable to a spate of lawsuits. If the White House insisted on extending the act to those over 70, the business lobbyists said, they would push for other amendments that would weaken the law's protection for older workers.

As part of this effort, the law firm of Pepper, Hamilton and Scheetz (no connection to Rep. Pepper), which frequently defends corporate clients in age-discrimination lawsuits, began circulating suggested changes in the law on Capitol Hill.

The administration's reported shift became apparent on May 12, when Labor Undersecretary Malcolm Lovell canceled a scheduled meeting with Rep. Pepper on the issue. Lovell instead called Pepper to say that the administration's position was still being worked out, according to congressional aides.

Lovell reportedly said he would like to convince Pepper of the merits of a more narrow approach that would bar only arbitrary dismissals of workers over 70. This approach would allow an employer to refuse to hire a person over 70, or to demote or reduce the wages of an employe over 70 purely on the basis of age.

Ed Howard, general counsel of the National Council on Aging, called that position "ludicrous. It means you have the right to a job only if you already have one. If I walk in the day after my 70th birthday, it's perfectly legal for you to turn me away." Pepper also rejected the proposal on these same grounds.

A well-placed administration source conceded that there were "some inexact communications" after Reagan's Rose Garden statement. According to this official, Reagan's decision was always limited to the narrow question of forced retirement. "That's all the decision was, ever," he said. "There were some people at the departmental level who didn't understand that and communicated it improperly. It comes down to that.... They goofed. They just didn't have it right."

This official emphatically denied that pressure from business groups forced the change. "I could understand how someone else would draw that conclusion," he said, "but it really isn't the case."

The government estimates that abolishing the mandatory retirement age would add only 200,000 employes to the work force over the next two decades, but the issue has political importance beyond its immediate impact.

Howard said Reagan timed his announcement for April 2 because 3,000 delegates were in town for a conference on aging at the Sheraton Washington. They heard the same promise from Vice President Bush. "It enabled Reagan to make some points with a constituency that was not too happy with his budget," Howard said. "Now we want the administration to deliver on its commitment."

A Labor Department official who first briefed Congress on the issue also contended that Reagan's position initially had been misconstrued and improperly explained to Congress.

But aides say that Heinz, Pepper and Sen. Orrin G. Hatch (R-Utah), chairman of the Labor and Human Resources Committee, all believe that the White House has backed away from its original proposal. And this, they say, has put the legislators in the difficult election-year position of either having to oppose a strong business lobby or having to vote against additional protection for elderly workers.

The business lobbyists have picked up support from some Republicans, such as Senate Labor subcommittee chairman Don Nickles of Oklahoma. An aide said Nickles "doesn't like the idea that someone who's 75 or 80 can walk in and say he wants a job and you can't say, 'Well, I just think you're too old.' "

Michael Romig, an official with the Chamber of Commerce, said that businesses are particularly concerned about the law's requirement that older workers may demand jury trials if they take an employer to court. Jury trials are not required in cases involving race or sex discrimination. He said this provision, adopted in 1978, has encouraged much expensive litigation, often prompting companies to settle out of court.

Romig said the Chamber also wants to eliminate a section of the law that awards a successful plaintiff damages of twice the amount of his lost wages. If Congress wants to extend the law to those over 70, Romig said, "many companies want to insist on a quid pro quo" of eliminating jury trials and double damages.

The Pepper, Hamilton law firm has been especially active in trying to change the law. One attorney with the firm, Jay Urwitz, worked on the 1978 amendments to the act as an aide to Sen. Edward M. Kennedy (D-Mass.). Walter B. Connolly Jr., the partner in the firm in charge of labor litigation, is the brother of Michael Connolly, general counsel of the Equal Employment Opportunity Commission, which has the authority to bring anti-discrimination suits.

Connolly said the firm has been lobbying on behalf of some 10 to 20 "top" corporations who want the law changed. The administration's position "gives nothing to industry," Connolly said. "Industry will only be happy if the double damages and jury trial are removed."

Under the change sought by supporters of the aging, Howard said, businesses still could dismiss elderly workers who are incapable of performing a particular job. "We're not trying to foist a bunch of feeble septuagenarians off on American industry," he said.