It's been a long, long time since any piece of legislation passed the Senate 62 to 10 and the House by 391 to 6, but that's the way the bill to wipe out mandatory retirement at age 65 sailed through Congress. Still, it's only the beginning of what's ahead.

The legislation, which testifies to the growing might of our "senior citizens," raises the compulsory retirement age from 65 to 70 in most private industry and removes such restrictions altogether from virtually all federal workers.

Before the president's signature was dry on this legislation, however, additional bills were being drafted to eliminate mandatory retirement at any age for everybody, either in private or public employment.

The new law is a move in the right direction, but it still isn't completely in step with what the polls indicate is the prevailing sentiment of older American workers, most of whom early retirement, but oppose being arbitrarily pushed out before they are ready to quit and while they are still in good health.

Not only Congress, but the courts and the state legislatures as well are responding sympathetically, with California setting the pace. It is the first state to abolish mandatory retirement at any age, and it's being watched closely to see how the change works out.

For years, opponents of the reform have been predicting dire consequences, such as increasing the rate of unemployment by curtailing the hiring of new workers, slowing promotion for younger employees, upsetting personnel and pension systems and risking a drop in productivity.

Little hard evidence has been produced to back up these fears, especially the notion that an army of elderly employees will now elect to go on working indefinitely instead of retiring, as now, at 65 or 70. The trend in recent years has been just the reverse. In 1966 the share of workers retiring before 65 under Social Security was 38 percent. Since then it has climbed every year, reaching 66 percent in 1976.

A national survey just completed by U.S. News finds that "most will go right ahead with plans to bow out as soon as they can." The Inland Steel Company, whose union contract bans forced retirement, reports that only 41 out of 18,243 production workers at its Chicago mill are over 65 years old.

There never has been any scientific reason for settling on 65 as the normal retirement age. American picked it up from a German pension system established 100 years ago, and it became a national fixture in the United States when it was adopted by the Social Security Act of 1935. Since then, life expectancy has increased and is still climbing.

Much of the opposition to the new legislation came from the nation's colleges and universities, which were concerned about senility. Guaranteeing "Professor Dry-As-Dust several more years of lectures from yellowing notes" is not an appetizing prospect, said Theodore Lockwood, president of Trinity College in Hartford, Conn.

Congress, however, was more impressed by the experience of the Hastings College of Law in California, which in 1950 adopted a rule against hiring anyone younger than 65 as a fulltime professor. Originally this policy was adopted for emergency reasons, but it produced such a distinguished lineup of elderly scholars that Roscoe Pound, dean emeritus of the Harvard Law School, rated it as the strongest law faculty in the country, even though its average age was 72.

Another reason for congressional sympathy is that 19 senators and 37 representatives are over 65. Senator sI. Hayakawa (R-Calif.) was in his 70s when he ran for election in 1976. Challenged about his age, he said: "Before World War II in Japan they killed off all the older politicians. All that were left were the damn fools who attacked Pearl Harbor. This country needs older statesmen, too." The voters agreed.

In a recent Roper survey, 61 percent of those polled said that, if they had a choice, they would like to have a parttime job when they retire: Only 26 percent said they would rather not work at all.

This seems to fit in with a development that Garson Kanin, the playwright, discusses in his wife and witty new book, "It Takes a Long Time to Become Young." One of the most reasonable proposals, he says, is the idea of gradual retirement. He describes it as:

"A phased-out work program; workers deciding at some given point to work shorter hours, to take longer vacations, thus allowing both the employer and the employee to adjust to eventual retirement."

Kanin quotes the comedian Joe E. Lewis as saying, "You're only young once - and if you work it right - once is enough." George Bernard Shaw said it even better. "Youth," he quipped, "is much too precious to waste on the young."