What was going on when I was born?

New York City's population shifted as dramatically during the decade of the 1970's as at any time in the history of the city. That is the conclusion of demographers, planners and other city experts who have now had several months to study the first reports from the 1980 census. Markedly Fewer Children The figures provide a profile of a shrinking populace that nevertheless includes more old people and young adults, but markedly fewer children and teen-agers. The figures show what may have been the largest exodus of New Yorkers in the city's history. At the same time, they show a city that has become a magnet for a new wave of immigrants from Asia and Latin America.

Never mind that Labor Day has passed, that temperatures are just sneaking into the 70's, that department stores are filled with bulky sweaters, fur-lined gloves and argyle socks. David Elliot was oiled up and lying on the beach here today - he even went swimming - and though he certainly wasn't fighting for space to stretch out his towel, he wasn't alone either. ''As soon as Labor Day comes, most people put on winter clothes and think the beach is over,'' said Mr. Elliot, his manner showing clear contempt for this sort of thinking. ''Actually, now is the best time.''

''HUT-HUT, pass the potatoes.'' Scott Brunner smiled. It was a level-headed, intelligent smile, not overemotional, a smile in the manner he quarterbacks. When your father is a football coach and your mother a former college basketball player and women's field-hockey referee, dinner might be expected to sound like a locker room with pep talks between courses. ''It certainly wasn't like that,'' said Brunner, who will lead the Giants against the Green Bay Packers tonight in the first Monday night game at Giants Stadium. ''My father was a father. He wasn't a coach.'' ''Hut-hut, pass the salt.'' John Brunner laughed. ''Scott overcame it all,'' he said. If his son, the Giant quarterback, can be described as born and bred to play football, it wasn't because John Brunner, now the offensive backfield coach of the Detroit Lions, was in the business, or because the family sat around the table and talked of post patterns and weakside linebackers.

Dave Winfield sat in the clubhouse at County Stadium before today's game discussing the challenges remaining for the Yankees, a team already eliminated from this year's division race. He cited personal statistics, preparation for next season and performing the spoiler's role in the American League East pennant race. ''We're not even doing a good job of that,'' Winfield said of the final challenge. Then the Yankees went out and ran their losing streak to eight games with a 14-1 loss to the Milwaukee Brewers.

Twenty years after the Soviet Union touched off a major confrontation with the United States by stationing nuclear missiles in Cuba, six key Kennedy Administration officials say that the decisive military element in resolving the crisis was the availability of superior American conventional forces, not superior American nuclear forces. The former officials express their views and conclusions about what became known as the Cuban missile crisis of October 1962 in a two-page essay in the current issue of Time magazine. In a pointed allusion to the current debate about military posture, tactics and strategy, they write: ''The decisive military element in the resolution of the crisis was our clearly available and applicable superiority in conventional weapons within the area of the crisis. No one of us ever reviewed the nuclear balance for comfort in those hard weeks.

''With a soul filled with bitterness and deep pain I have learned the news of the horrendous massacres committed in the Palestinian camps in Beirut.

Big multinational corporations are coming under a far-reaching attack by nations that want to see them more strictly regulated. The attack, originating both in the European Common Market and in the Group of 77, the caucus of third world countries in the United Nations, reflects deepening concern over the wealth, power and global reach of these companies. The aim of the attack, in each case, is to make large companies more accountable to their employees and to host governments. There are also efforts to force them to do more to advance the industrialization of the third world.

Jason Siegel of San Diego would go to the zoo, Richard Somers of Miami would get his shoulder pads out of the attic and Rose Marie Hoyson of Carnegie, Pa., did not know how she would spend her Sunday afternoons and Monday nights. They were among millions of fans who watched yesterday's National Football League games while contemplating the prospect that tonight's nationally televised contest between the Green Bay Packers and the Giants at Giants Stadium could be the last pro game they would see for some time this season. The fans seemed no more happy about a strike than the N.F.L. Players Association's executive committee, which might vote for one today. Almost all the players who would be striking after only two games this season said they hoped for a last-minute resolution. But they, like the fans, generally felt resigned to the likelihood of a strike.

Peking has granted Atlantic Richfield permission to drill for oil offshore. The contract, the first signed with an American oil company, took four years to negotiate. It covers 3,500 square miles near Hainan Island that Atlantic Richfield's top officers say has 20 promising geological structures and is ''probably the longest unexplored virgin coastline in the world.'' Santa Fe International is Arco's partner in the contract. (Page A1.)

In the pharmaceutical industry, no area of research is more compelling than the battle against cancer. But efforts to develop effective cancer drugs have often given way to the realities of the bottom line. As a result, many major drug companies have played a relatively small role in the expanding field of chemotherapy, or the use of drugs to kill cancer cells. ''It's a risky business because the sales potential is small and the expenses are high,'' said Saul Shepartz, deputy director of the division of cancer treatment of the National Cancer Institute. Mr. Shepartz's organization, a Federal agency, has used public funds to finance the development of a large share of the cancer drugs now available. Marketing Successes Gradually, however, the economics of cancer research are changing. Far more than in the past, major drug companies are seeing a potential for profit in developing cancer drugs. In large part, this results from the success of a handful of new cancer treatments that have been brought to market in the past three years.

International Sending peacekeeping forces to Beirut to prevent new killings and strengthen the Lebanese Government was discussed by President Reagan and his top aides, the White House said. A senior official said the Administration was leaning in favor of a redeployment to Beirut of about 2,000 of the 7,000-member United Nations peacekeeping force now in southern Lebanon. (Page A1, Column 6.) Lebanese forces entered the camp in west Beirut where Lebanese Christian militiamen murdered scores of Palestinian men, women and children. As troops and civil defense volunteers moved to establish order and to remove the mounds of corpses, new information emerged about the extent of the massacre of civilians and the role played by Israeli forces. (A1:5.)

Senior Israeli officials said today that they never imagined that the Lebanese Christian militiamen would massacre scores of Palestinian men, women and children in west Beirut. They added, however, that they knew in advance that the Phalangists were planning to enter the refugee camps as part of an overall plan to root out remaining guerrillas. The Israeli Cabinet, meeting in emergency session, issued a statement tonight calling allegations of Israeli responsibility a ''blood libel.'' It said it felt ''deep grief and regret'' over the mass slayings and declared, ''No one will preach to us ethics and respect for human life, values of which we have educated and will continue to educate generations of Israeli fighters.'' (The Israeli radio said the Cabinet, apparently in response to a new Security Council resolution, decided to permit the entry of United Nations observers into Beirut, Reuters reported. The resolution called for 40 more observers to join the 10 alreadt there.)