What was going on when I was born?

The Internal Revenue Service is unpopular at the best of times, but lately it has sunk to some sort of new low among waiters, waitresses and managers at restaurants and bars throughout the country. The reason: a new regulation aimed at squeezing greater tax revenue out of tips. Employers are complaining about the regulation's complexity and about the costly new administrative burden they must shoulder. The employees are unhappy about a requirement that a fixed rate be used in estimating their tips, a fluctuating but critical supplement to their income. Both groups insist that an injustice is being done. ''The whole thing is really a can of worms,'' said Ronald A. Sarasin, director of government relations for the National Restaurant Association in Washington. ''And it's not fair.''

More than 2,000 Poles packed into a Roman Catholic Church in this industrial suburb of Warsaw tonight for a protest mass that represented a challenge to the authority of the Polish Primate, Jozef Cardinal Glemp. The reason for the protest was Cardinal Glemp's decision last week, under what church officials said was intense Government pressure, to transfer the Rev. Mieczyslaw Nowak, an outspoken supporter of the outlawed Solidarity trade union founded by Lech Walesa. Father Nowak was head of the St. Joseph the Worker parish in Ursus, site of a sprawling tractor factory that is one of Poland's largest industries and was once a Solidarity stronghold. A Division Within Church The protest mass, an extraordinary event in this deeply Roman Catholic nation where the authority of the hierarchy has traditionally been respected, appeared to reflect a growing division between Cardinal Glemp and many of the younger priests and laity Since the imposition of martial law in December 1981, local churches have become sanctuaries for the remaining spirit of Solidarity, reflecting what has been seen here since the end of the 18th century as the church's role as the protector of the Polish nation.

The Italian contingent of the multinational force completed its withdrawal from the Lebanese capital today. The departure of all but 100 members of the 1,385-member Italian force left about 1,300 American marines and 1,250 French soldiers protecting areas in Beirut. The remaining 100 Italians were staying behind in the port area to ''tie up loose ends,'' according to an Italian military spokesman. Marine officers said tonight that they had received orders from the Pentagon to begin loading up their combat forces. They said all the men had been told to ''get their stuff together'' and would probably begin pulling out of the bunkers Tuesday by helicopter and landing craft. They said the transfer to their ships would probably be spread over seven or eight days.

RESEARCHERS can now demonstrate that certain crucial areas of human intelligence do not decline in old age among people who are generally healthy. Moreover, although some other aspects of intelligence do dimish, the decline is relatively inconsequential and has been exaggerated in the past, the experts assert. The new research challenges beliefs long held by scientists and the public and suggests that, among people who remain physically and emotionally healthy, some of the most important forms of intellectual growth can continue well into the 80's. It also suggests that declines in intelligence can be reversed in some instances and that earlier notions about the loss of brain cells as a person ages were in error. This more optimistic view of the mental capacities of the aged emerges from a broad range of current studies, from recent literature in the field and from interviews with gerontologists, psychologists and experts in related health sciences.

''The future has been postponed, not canceled.'' - James J. Whalen, president of Ithaca College, on an unexpected surge in student applications. (C10:3.)

Walter F. Mondale won a commanding victory in the Iowa Democratic caucuses today, and John Glenn, running far back in the eight-candidate field, took a beating that threatened to cripple his Presidential candidacy. In a surprising performance based on his effective late campaigning here, Senator Gary Hart of Colorado finished second in the initial electoral test of the 1984 campaign. Not far behind Mr. Hart was former Senator George McGovern of South Dakota, whose candidacy here was initially regarded as a nostalgic curiosity. Mr. Hart hailed his showing as a breakthrough. But Mr. McGovern's unexpectedly high finish also seemed to mark something of a political resurrection for the man who was the Democrats' nominee in 1972.

JUST when critics of American education are looking with envy to Japan's tough education standards, Japanese educators appear worried that their schools may be too tough. They are even turning to American school reformers for suggestions. When Ernest L. Boyer arrived in Tokyo for meetings with education authorities earlier this month, he was surprised to find translators at work on a Japanese edition of ''High School,'' the report on Secondary Education in America. The report, issued last year by the Carnegie Foundation for the Advancement of Teaching, which Dr. Boyer heads, prescribes extensive reforms. And while it did not join those who blame American schools for Japan's economic victories, one of its key points was nevertheless that educational standards in American schools need to be raised if the United States is to compete in foreign markets.

A column by Dave Anderson in SportsMonday incorrectly characterized the accomplishment of Phil and Steve Mahre Sunday in the Winter Olympics at Sarajevo, Yugoslavia. The first set of brothers to finish first and second in an Olympic event were Platt and Ben Adams of the United States, in the standing high jump at the 1912 Summer Games at Stockholm.

The beer and whisky flowed freely, followed by shrimp cocktail and heaping plates of steak and lobster. Last Thursday's party was for traders from the floor of the New York Stock Exchange to celebrate the end of ''when-issued'' trading in the stripped-down American Telephone and Telegraph Company and the seven regional holding companies it had spun off. ''I'd have to say we did all right,'' Peter Larkin, a floor broker with Paine Webber Inc., said as about 100 usually competing traders mingled. ''I'd say it carried some firms over a bad period.'' Nonetheless, it was only these professional traders - the arbitragers who can take advantage of small price fluctuations - who cleaned up in the three months of when-issued trading that ended last Wednesday. The windfall from the A.T.& T. breakup that many had predicted for securities firms serving both individuals and institutions has so far failed to materialize.

TUESDAY, FEBRUARY 21, 1984 The Economy January's strong economic data have led many analysts to again forecast vigorous growth. But some fear that interest rates may be forced up by demand for credit from industry (to expand output) and from the Government (to pay for deficits). (Page D1.)

After more than a week of gradually falling prices in the credit markets, investors have still not bought up the $16.25 billion in Treasury securities auctioned earlier this month. The weak demand has led some market participants to worry that major dealers might begin cutting prices aggressively as they tire of waiting for demand to materialize. Since the Treasury auction of $16.25 billion of 3- , 10- and 30-year securities, the market has been struggling. Investor caution was evident at the auctions on Feb. 7 and 9, but many market participants were optimistic that buyers would come forward in the days following. But the buyers have not come forward.

The Italian force completed its pullout from Beirut. The departure of nearly all the 1,385 Italians left only about 1,300 United States marines and 1,250 French troops to protect areas of the Lebanese capital. Marine officers said the Americans would probably start withdrawing to ships offshore today and complete the pullout in about a week. (Page A1, Col. 6.) The possibility that U.N. troops who have been stationed in southern Lebanon since 1978 will be sent to Beirut to replace the departing Western troops seems unlikely now, according to United Nations officials. (A1:4-5.)