More than a third of young adults have delayed going to college because of difficult economic conditions in the United States, says a report released on Wednesday by the progressive nonprofit organization Demos and the advocacy group Young Invincibles. Exactly half of 18-to-24-year-olds reported less than $5,000 in total debt; 8 percent owed more than $25,000, according to the report, “The State of Young America,” which also collects data on college-completion rates, tuition and student loans, and employment and health insurance.