President Carter signed into law yesterday legislation that will prohibit employers from forcing most employes to retire because of age before they are 70 and will eliminate mandatory retirement altogether for most federal employes.

In a signing ceremony in the White House Rose Garden, the president implicity endorsed extending the new law so that most Americans, not just federal imployes, cannot be forced to retirely solely because of age.

'This legislation will remove any age limit on employment in the federal government,' he siad. 'And we hope this will be a good example for the rest of the nation to emulate . . . We are encouraging state and lcal governments to follow the lead of the federal government.'

Rep. Claude Pepper (D - Fla), whose district includes a large number of elderly voters, has already announced that he will intorduce legislation to eliminate mandatory retirement for almost all workers.

The legislation signed yesterday does not affect the ate at which people are leigible to teceive Social Security benefits.

The rtirment provisions, a revision of existing law prohibiting compulsory retirement before age 65, are effective Sept.30 for federal employes and next Jan. 1 for state and local government and employes of private firms. They state that private employers with 20 or more workers and state and local governments may nt require an employee to retire before 70 solely because of age.

An exception in employes covered by collective in bargaining agreements that contain mandatory retirement clauses. They would be exempted until the agreement runs out or Jan. 1, 1980, whichever comes first.

The retirment provisions also do not apply to executives and 'high policymaking employes' who are eligible for an annual pension of $27,000 or more. These employes could still be required to retire at 65.

Also exempt are such federal employes as law enforcement personnal firefighters, air traffic controllers, foreigh service officers and certain CIA personnel.

Carter used the signing ceremony to address another subject of interest to older Americans - the Social Security system. He praised Congress for 'courageously, at some political cost,' voting to increase Social Security taxes. In effect he was criticizing the current congressional efforts to roll back the tax increase.

In another development, the White House released a transcript from an interview of the president Wednesday on the television program 'Black Perspective on the News.'

Defending his record in office, Carter said in the interview that he believes he has earned the 94 percent of the black voter he received in the 1976 election and that the adnministration's record in dealing with blacks is not subject to 'legiimate criticism.'

The president also said that 'to a substantial degree the outcome of cogressional elections this year will be a measure of my administration's approval among the voters.' He balmed the decline in his popularity on the fact that 'a lot was expected of me' when he came into office, saying that if former presidents Nixon and Ford had done as much as we have done already, there would have been a tremensous beneficial and apprecia.

Earlier yesterday, the president visited a White House breadfast meeting of business, labor and political leaders on tax revision.

'There is a general sense that I share that the present tax system is not fair, it's not equitable, it's not equitable, its not simple' he told the group.

Officials announced that Carter is calling a White House Conference on Small Business to be held here in the fall of 1979 and that he will address a Cook County Democratic fund - raising dinnerinChicago May 25.