HCR Update from Mark Sanna: President Obama announced Thursday that eight million people have signed up for health insurance under the Affordable Care Act, including what the White House said were a sufficient number of young, healthy adults, a critical milestone that might counter election-year attacks by Republicans on the law’s success and viability. 35% of those who signed up through the federal health-insurance exchange were in the coveted under-35 demographic, Mr. Obama said. The participation of younger, relatively healthy people is needed to balance out the cost of medical claims from older and sicker ones.