Jimmy Stewart, the 76-year-old movie actor and friend of President Reagan, gazed at the crowd of senior citizens and said he, for one, was delighted to be old even though the time comes when the candles cost more than the cake. But he feels sorry for those who no longer celebrate birthdays -- he himself is proud to tell people when they ask his age that he's 40 and has been for decades.

Inaugural festivities did not overlook the old, and the Salute to Older Americans was an official event on Saturday in the huge marble lobby of the Library of Congress. People leaned over the marble stair balustrades and an orchestra played as about 300 sampled the afternoon buffet of egg rolls and chicken wings, meatballs, raw vegetables and cheeses served from so many tables that nobody had to wait. The tea was hot and strong, and the wine had to compete with a popular champagne punch. From time to time waiters appeared holding silver tazzas loaded with admirable cookies.

The sound system annoyed some feisty seniors, one of whom hollered, "We can't hear, and it's not because we're too old."

Hugh O'Brian, 54, long the star of "The Life and Legend of Wyatt Earp" on television, with a long list of charitable undertakings to his credit, introduced Margaret Heckler, 53, secretary of health and human services, as a "super-wonderful woman," and the Cabinet secretary spoke briefly to the point that the American standard of living and safety was won by those no longer young. She also pointed out that her budget of $319 billion is exceeded only by the budgets of the American and Soviet governments. Her goal as secretary, she said, is to reinforce a sense of independence in the old, adding that age is largely a state of mind and complimenting Reagan for chopping more wood "than Abe Lincoln ever did."

Girl and Boy Scouts lined the stairs to the reception room, though hardly any oldster accepted assistance. The guest list was composed largely of people in public and private agencies that deal with the old. Heckler said the party was made possible by the Silver Pages division of the Southwestern Bell phone company.