Nearly 3.3 million people have signed up for health insurance through the marketplaces established by President Obama's health care law, and about one-fourth of them are young adults, the administration said Wednesday. The administration reported a modest uptick in the enrollment of young adults, a group avidly sought by insurers because they are usually healthier and need fewer costly medical services. In a new report on enrollment, the administration said that 1.9 million people had selected health plans in the federal marketplace from October through January, while 1.4 million chose plans in state-run insurance exchanges.