AFTER LONG SILENCE (350 pp.)Robert GutwilligLittle, Brown ($4).
Younger generationslost, silent, or beatare presumed to share one quality: youth. After Long Silence, a first novel by Manhattan's 26-year-old Robert Gut-willig, is symptomatic of a recent fictional tendency to portray the yo.ung as prematurely aged and jaded. A scabrous episode early in Author Gutwillig's book suggests its Sagantiquated antics: "Males and females were naked to the waist. The couples seemed to be licking each other's shoulders, necks, and chests . . . Each couple had a little can with holes in the toplike a large salt cellarand from time to time they would...