Changes mandated by the federal Age Discrimination in Employment Act, allowing most employes to stay on the job until aga 70, have not slowed in any significant way the trend toward retirement prior to age 65.

But this pattern could change dramatically under the impact of double-digit inflation.

These are among the conclusions of a survey of 100 major employers by Charles D. Spencer & Associates Inc., a Chicago publisher of information about employe benefits. The companies employ 1.5 million persons and represent a cross-section of large corporations.

During the first six months of this year, with the amended law in effect, 70 percent of employes who retired were younger than 65. This is a higher percentage than during 1978, when 62 percent of employes who retired were under age 65 at the same companies.

Other findings of the survey:

Last year, 76 of the 100 firms required employes to retire at 65. By June 30, all had eliminated the retirement age or raised it to 70.

Also in 1978, employes who were 65 or older made up less than one quarter of one percent of the active work force in the 100 companies. As of June 30, the percentage was exactly the same even though virtually every worker who retired at age 65 in 1979 could have continued working until age 70. w

At the 100 companies in 1978, 32,372 persons retired. Of the total, 16 percent received disability pensions, 62 percent were younger than 65, 19 percent were 65 and 2 percent were older than 65.

John Cisarik, an associate editior at the Chicago company, cautioned that retirement patterns found in the new survey could change in coming years.

"It is our feeling, and the feeling of many respondents to the survey, that continued inflation could change things drastically. Persons who retired this year most likely planned their retirement at least a few years ago, and the change in the law came after they had already decided to retire," he commented.

Employes reaching 65 in the next few years, aware that they can continue to work, may well decide to postpone retirement, he stated.