For years, younger Americans have been more optimistic about the job market than older Americans, even through the depths of the Great Recession. But in an abrupt shift, a new poll released Monday finds young people’s confidence has plummeted over the past two years — while their elders remain more upbeat. The gap between young and older Americans’ views of the job market now is greater than in any other country among the 141 surveyed, according to the Gallup World Poll . In the United States,