At 90 years old, Loretta Corcoran may be Washington's oldest professional model. She's the cousin of the founder of the Corcoragn Gallery of Art and the Riggs National Bank. For 28 years she worked as a secretary in the Army surgeon general's office at the Department of Defense. Then her great-niece began a modeling agency called Anne Schwab's Model Store, and when the American Association of Retired Persons needed a kindly looking senior citizen for a slide show and brochure, Schwab asked her great-aunt if she wanted to do it.You bet, she said. So she played the role of a little old lady who is swindled out of her savings by con artists. (Needless to say, the specimens of beefcake - male models - in this photo weren't around to protect her, or there would have been a few sorry con artists.) How does Corcoran, who was paid $60-an-hour while working, like the fast life of a model? "Easy money," she says. She's ready for other jobs but was recently passed over for a shooting assignment at a nursing home. She was told she didn't look old enough. CAPTION: Picture, no caption, By Bill Snead