Higher education made up some ground last fall and in the spring semester, largely as community college enrollment began to rebound, National Student Clearinghouse Research Center data showed. But the trend for four-year colleges remains worrisome. Even without growing concerns about the cost of college and the long-term burden of student debt, the pool of young adults is shrinking. Birth rates fell during the Great Recession of 2007 to 2009 and never recovered. Now those smaller classes are preparing to graduate and head off to college. “It’s very difficult math to overcome,” said Patrick Lane, vice president at the Western Interstate Commission for Higher Education, a leading authority on student demographics.