NO ONE REALLY KNOWS what the effect will be of the legislation working its way through Congress to raise the mandatory retirement age from 65 to 70. But that isn't slowing the bill down. The House waved it through in late September by vote of 359 to 4 and the Senate passed its version last Wednesday by 87 to 6. Details remain to be worked out, and there is sharp disagreement over some of them, but it appears certain that a bill will emerge shortly that will make it unlawful for most, if not all, employers to compel workers to retire before they are 70.

The changes this legislation could work in the American economy and in the role of older peoplein society are enormou* s. Could is the key word, for despite the weeks of hearings on this and similar proposals, there are few facts on which to make reliable predictions.This bill's impact - on the unemployemnt rate the financing of pension plans, the productivity of workers and the re-adjustment in living patterns - depends on what thousands of inviduals decide to do. And no one has a handle on what those decisions will be. THe estimates are that anywhere from 5 to 20 per cent of those who become 65 each year will elect to keep on working - at least for a while - and the difference between those two numbers is sufficient to make a huge difference in calculating the effect of the bill.

The principal reason why the economids of this legislation have received so little public consideration is that the bill is widely regarded as a civil-rights measure. It is, fact, an amendent to the 1967 Age Discrimination in Employment Act, which now bars employers from the discriminating against anyone because of age between 40 and 65. But it seems to us - and to many of the bill's supporters - that this is the fore-runner of legislation to bar mandatory retirement at any age. That idea, as well as this bill, has an unusual array of supporters ranging from liberals, who see mandatory retirement as a matter of discrimation, to conservatives, who regard it as an interference with freedom of choice. Public-opinion polls indicate that a vast majority of Americans, even those who think most workers ought to stop work when they are 65, dislike mandatory-retirement systems.

With that kind of support, opposition to this legislation by some business and labor groups never had a chance. They can no more produce reliable data to support their argument that the bill will harm employers and young workers than can its proponents demonstrate that the bill will ease the financing worries of the Social Security system and of some pension funds. In that sense, the legislation has many aspects of a social experiment. Consider these possibilities - if substantial numbers of those reaching 65 chose to keep on working.

Personnel managers may be forced to face up to the problem of the declining productivity of some older workers - a problem that is now often ignored because "he'll retire in a couple of years, anyway."

Young people may discover that the job market is tighter than it otherwise would be because older workers aren't retiring.

The cost of corporate pension funds may come down while health-insurance costs go up.

Employers who want to get rid of older employees may find that the easiest way is to improve their pension plan so that early retirement is more attractive.

Employers may even discover that it is possible to arrange schedules so that older persons who really wnat to work only part-time or part of a year can be accomodated.

Salesmen may discover a whole new market out there of people over 65 who suddenly have more money to spend than 66-year-olds had in the past.

You can make a case, if you want to, that the uncharted waters into which this legislation takes us ought to be explored more carefully before they are entered. But that is not going to happen. Moving up the mandatory retirement age to 70 is an idea whose time has come. Our only suggestions to the congressional conference committee are that it give employers enough time to adjust to the new rules and that it seriously consider eliminating all exceptions; both houses have picked out groups where the age limit of 65 would still apply. There is nothing magical about either number and there is something arbitrary about trying to apply one number to one kind of worker and a second to another. Anyway, both numbers will disappear in a few years if this experiment turns out well - as we suspect it will - and neither will be particularly relevant if it turns out badly.