Demographers say it will finally arrive nationwide in the fall of this year. That’s when recruiting offices will begin to confront the long-anticipated drop-off in the number of applicants from among the next class of high school seniors. But the downturn isn’t just a problem for universities and colleges. It’s a looming crisis for the economy, with fewer graduates eventually coming through the pipeline to fill jobs that require college educations, even as international rivals increase the proportions of their populations with degrees.