“I know everything,” the composer Ned Rorem said on a recent afternoon, “but I remember nothing.” Mr. Rorem — elder statesman of American art song, prolific prose writer, pioneer of gay liberation — was exaggerating. Now 95 years old (as of Tuesday), he is more forgetful than he used to be. But he actually remembers quite a lot. Give him the right prompt, and he can dish at length on nearly every cultural luminary of the past century — he’s known almost all personally — and reminisce about Picasso’s Paris or the New York City of $25 rents. He’s a...