Young adults are the nation’s largest group of uninsured — there were 13.2 million of them nationally in 2007, or 29%, according to the latest figures from the Commonwealth Fund, a nonprofit research group in New York. They borrow leftover prescription drugs from friends, attempt to self-diagnose ailments online, stretch their diabetes and asthma medicines for as long as possible and set their own broken bones. When emergencies strike, they rarely can afford the bills that follow.