What was going on when I was born?

EVER since car makers hit on the notion of the annual model change, planned obsolescence has been a standard marketing aid. The idea, of course, is to make the owner of last year's model feel stodgy and out of fashion so he'll rush out and buy the very latest. A kindred philosophy prevails in stereo. Each year a new generation of sound gear beckons from the shop windows, tempting the aficionados. But temptation should be distinguished from real need. In these economically wobbly times circumspect listeners are likely to ask themselves whether their sound systems really warrant replacement.

Here at South Commack, a large suburban high school where college has always been synonomous with sacrifice, more students than ever are planning to say no this year to elite private colleges and register instead at public institutions. Because of cutbacks in student aid and deepening economic uncertainty, close to three dozen seniors in this middle-income community have come to the conclusion that it will be financially impossible to attend Ivy League and other select colleges that have accepted them, according to the school's guidance office. Beverly Ballaro, for example, the salutatorian, who has spent an academic lifetime hoping to attend an Ivy League school, was recently accepted by Yale. Instead she will attend Rutgers, New Jersey's state university, which is considerably less costly.

the mansions in which interior designers display their talents - are a telling barometer of the design profession at a given moment. The Kips Bay Boys' Club Decorator Show House '82, one of the most prestigious, is no exception. As in years past, the Kips Bay Boys' Club invited a group of leading interior decorators to participate, with rooms assigned by lottery. It is the 10th annual event for this charitable organization, which helps disadvantaged children. But while past Kips Bay show houses tended to reflect new ideas and directions, this year's, which opened to the public yesterday, is more traditional.

A chart on Sunday gave an incorrect figure for the University of Central America's estimate of the number of civilians killed in El Salvador in 1981. The correct figure is 13,229.

A Federal judge ruled yesterday that New York State was failing to comply with reforms pledged in 1975 in care for the mentally retarded. He said he would name a special master at state expense to supervise the reform program. The judge, John R. Bartels of District Court in Brooklyn, charged that sanitation at the Staten Island Developmental Center in Willowbrook ''presents a serious health hazard'' to about 800 residents remaining under state care there. Life for them, he said, is ''sterile, dreary and one of enforced idleness.'' At least one patient, the judge said, ''choked to death as a result of being served an improper diet.'' Overall conditions, he said, had ''materially deteriorated'' since the State Legislature removed funds from a review panel in 1979.

''Exercise is the only thing that really keeps your mind clear when you get older.

The advertising column in Business Day last Monday incorrectly credited the creation of the advertising campaign ''Put a Tic Tac in your mouth and get a bang out of life.'' It was by Chalek & Dreyer.

Negotiations between President Reagan and Democratic Congressional leaders over how to control the deficit fell apart this afternoon in disagreement. Participants in a three-hour meeting in the Capitol said there was now almost no chance that the budget stalemate could be resolved through a bipartisan compromise. Thomas P. O'Neill Jr., the Speaker of the House, said the negotiations had failed because of a ''basic philosophical disagreement'' that emerged between him and Mr. Reagan at what a participant said was a sometimes heated bargaining session in the ornate President's Room of the Capitol. The task of putting together a budget for next year now falls to the leaders and committees of Congress. A result, according to Administration officials, is likely to be more uncertainty than ever over the size of the deficit in the next three years, which, in turn, could create turmoil in the money markets, continue high interest rates and hamper economic recovery. (Page B11.)

Officials of New York State and the Army Corps of Engineers said yesterday that new studies would be conducted to assess how much environmental damage would occur if the Westway highway project was built. State officials said they hoped the review could be conducted by the fall, but corps officials said the studies, which would include public hearings, could take a year or longer. After the studies are completed, the corps would decide whether to grant a second permit that would allow a stretch of the Hudson River off lower Manhattan to be filled for the project. ''A year is not outrageous - neither too short nor too long,'' said Everett Chasen, public-affairs officer of the corps in New York. He added that if the corps was required to do all the work itself, ''a year might not be long enough.''

Playboy Enterprises, the entertainment and communications company, announced today that Christie Hefner, a vice president of the company, had been promoted to president. She replaces Derick J. Daniels, who has resigned. Miss Hefner also said today that Playboy would divest itself of its casino and book publishing divisions, leaving it principally as a magazine publishing company. Miss Hefner, 29 years old, is the daughter of Hugh Hefner, who founded the company and holds 70 percent of the stock. Analysts and a Playboy spokesman described the move as an attempt by Mr. Hefner to reassert control over the company, which has been going through a transitional period under the leadership of Mr. Daniels, who joined the company in 1976 from Knight-Ridder Newspapers, where he was a vice president.

An article on the Washington Talk page March 11 incorrectly described the status of children of Isabel Letelier, widow of Orlando Letelier, the former Chilean Ambassador to the United States. Mrs. Letelier has been declared persona non grata by the Chilean Government, but her children have not.

The Commodity Exchange of New York, which had planned to begin trading futures contracts based on Standard & Poor's Composite Index of 500 stocks Thursday, was thwarted today when a Federal judge issued a temporary restraining order on a copyright suit by S.& P. The Comex had scheduled the start of trading in its new contract, which it calls the Comex 500, after the Commodities Futures Trading Commission granted permission today. The Comex went as far as announcing a news conference and champagne breakfast to mark the introduction. The restraining order, issued by Judge Milton Pollack of United States District Court for the Southern District of New York, came shortly after the futures commission gave its approval. Judge Pollack delayed trading of the contracts pending a hearing scheduled for next Wednesday.