Roberta Peters, 86, who debuted at the Metropolitan Opera at age 20 on five hours' notice and became a reigning soprano of her era, delighting audiences for decades with performances on stage, in commercials and on The Ed Sullivan Show , died Wednesday at an assisted-living facility in Rye, N.Y. The cause was Parkinson's disease, said her son Paul Fields. Ms. Peters' maiden entrance at the Met could scarcely have been more dramatic, even if it had been scripted by a librettist.