What was going on when I was born?

EVIDENCE is building that development and growth of the brain go on into old age. It was once thought that the brain was fixed by late childhood, according to innate genetic design. As long ago as 1911, however, Santiago Ramon y Cajal, a pioneering neurobiologist, proposed that ''cerebral exercise'' could benefit the brain. But a scientific consensus that the brain continues to bloom if properly stimulated by an enriched environment was long in coming. ''Over the last decade, neuroscientists have become impressed by the degree to which the structure and chemistry of the brain is affected by experience,'' said Floyd Bloom, director of the division of neuroscience and endocrinology at the Scripps Clinic and Research Foundation in La Jolla, Calif. The new research seeks to provide a more detailed understanding of that phenomenon.

President Reagan refused today to support the two central elements of the latest Senate budget plan: tax increases, including an oil import fee, and alternate-year adjustments of Social Security benefits to account for inflation. The announcement of the President's decision, awaited since Thursday, put an end to the Republican-led effort in the Senate to put together a plan to cut projected budget deficits by more than $300 billion over the next three years. ''Any chance for this year of getting a real, significant, reliable deficit reduction package is gone,'' said Pete V. Domenici, the chairman of the Senate Budget Committee. New Talks Due Today Directing the blame at the White House, he added: ''In this case there is not a will to address the huge deficit.''

An article July 18 about CBS public-service announcements to help find missing children cited broadcasters' efforts incompletely. Since last year, ''Good Morning America'' on ABC has broadcast information on missing children.

Sales at the nation's department stores and auto dealerships have suffered in recent months because of consumers like Tammy and Daniel Nelson. Last year and the year before, when things were going well at Mr. Nelson's job, the Chicago couple purchased several costly items, including a refrigerator and an air-conditioner. This summer, however, the Nelsons have cut their purchases because the metal-crafting company where Mr. Nelson works has reduced his schedule to four days a week. ''Business is slow and we're not rushing out to buy many things,'' Mr. Nelson commented recently.

President Reagan and the Soviet leader, Mikhail S. Gorbachev, each offered new proposals on the testing of nuclear weapons today. The White House announced that Mr. Reagan had invited the Soviet Union to send observers to witness an underground nuclear test at a Nevada test site, an Administration official said. Hours later, Mr. Gorbachev announced through the press agency Tass and on national television that the Soviet Union would impose a five-month moratorium on nuclear weapons tests. He said the moratorium would start on Aug. 6, the 40th anniversary of the United States bombing of Hiroshima, and would continue beyond Jan. 1 if the United States imposed a similar moratorium in the meantime. Meeting in Helsinki The two announcements came as Secretary of State George P. Shultz and the new Soviet Foreign Minister, Eduard A. Shevardnadze, arrived in Finland for a three-day meeting marking the 10th anniversary of the Helsinki accords on European security and cooperation. [Page A6.] Mr. Shultz told reporters traveling with him to Helsinki that the United States did not believe that ''it is in our interest to stop our testing program under these circumstances.'' He also expressed doubts about the sincerity of the Soviet moratorium.

MOST solid things are made of crystals, and for nearly two centuries scientists assumed that every crystal must have an orderly structure, its constituent atoms fixed at predictable, periodic positions within a lattice framework. But the discovery of a new type of crystal that violates some of the accepted rules has touched off an explosion of conjecture and research that may lead to the founding of a new branch of science. The finding has galvanized microstructure analysts, mathematicians, chemists, metallurgists and physicists in at least eight countries. According to one estimate, scientists around the world are now producing a paper a day relating to the discovery, and an end to this torrent of research is nowhere in sight. Whether the discovery will have practical consequences remains to be seen. But as one investigator put it: ''If this kind of crystal proves to have properties as peculiar as its structure, the stuff seems certain to find important uses. That's what one would expect in the field of condensed-matter physics.''

Peru has thrown down the gauntlet, challenging its creditors by becoming the first major country to take unilateral action to cap its debt payments to foreign banks. The move, which President Alan Garcia Perez announced in his inaugural address on Sunday, is important mostly as a symbol and a precedent. For international banks, the worry is not so much that Peru might not pay its debts - it already is months in arrears - but that other, bigger debtors like Brazil or Mexico might get ideas. No one thinks that other countries will follow Peru's lead very soon, limiting payments on foreign debt to a percentage of export earnings. But it is a fashionable idea in some Latin circles that countries should bargain harder with the banks and not accept cuts in living standards so that New York banks can report higher earnings. Mr. Garcia's statements reinforced this view.

International The U.S. and the Kremlin offered new proposals on nuclear weapons tests. The White House announced that President Reagan had suggested that Moscow send observers to witness an underground nuclear test in Nevada. [Page A1, Column 6.] Hours later, Mikhail S. Gorbachev, the Soviet leader, announced he would impose a one-sided, five-month moratorium on nuclear arms tests, starting on Aug. 6, the 40th anniversary of the United States' bombing of Hiroshima. He said the moratorium could be extended if the United States also halted nuclear weapons tests. [A6:3-6.] South Africa's President rejected a call for urgent talks with Bishop Desmond Tutu aimed at defusing rising tension over the mass arrests of dissidents. Bishop Tutu, a prominent South African black leader who won the 1984 Nobel Peace Prize, said the decision by President P. W. Botha ''demonstrates very clearly the crisis of this land.'' [A1:5.]

A picture caption in Metropolitan Report yesterday with an article about weekend activities around the city misidentified participants in the Great Bed Race on Third Avenue. They represented the Peerless Radio Corporation.

Margie Martinez, her T-shirt stained with sweat, emerged smiling and breathless from the sanitation workers' testing center at Floyd Bennett Field in Brooklyn yesterday. She had just finished lifting and tossing 147 garbage bags weighing a total of 2,975 pounds into simulated trash bins in 27 minutes. She scored 100 percent. Mrs. Martinez, 38 years old, of Queens, and about 20 other women who had taken a special course to prepare them for the physical test for uniformed sanitation workers spilled into the women's locker room, where they slapped palms and compared notes on how easy the test had been.

''They have gone on orbit. At the present time there is no plan to come home.

A pair of Siberian tigers at the Bronx Zoo attacked and killed one of their keepers yesterday in an enclosure that is part of the Wild Asia exhibit. According to Dr. William Conway, the director of both the Bronx Zoo and the New York Zoological Society, the keeper, 24-year-old Robin Silverman, violated the ''cardinal rule of animal care'' by entering the two-acre enclosure without knowing where the tigers were. ''It's one of those terrible human tragedies, an accident,'' Dr. Conway said. ''She was considered a very bright and competent person, very highly regarded by her colleagues here at the zoo. But when you're working with dangerous animals, you must be certain you know where they all are, and you don't go into the enclosure with the big cats.''