Medical issues rippled into other problems such as using all of their savings (43 percent), being unable to make student loan or tuition payments (32 percent), delaying education or career plans (31 percent), or being unable to pay for necessities such as food, heat or rent (28 percent).

"While the Affordable Care Act has already provided a new source of coverage for millions of young adults at risk of being uninsured, more help is needed for those left behind," says Commonwealth Fund Vice President Sara Collins, lead author of the report. "The law's major insurance provisions slated for 2014, including expanded Medicaid and subsidized private plans through state insurance exchanges, will provide nearly all young adults across the income spectrum with affordable and comprehensive health plans."

Young adults in low-income households are the most vulnerable to gaps in coverage. Seventy percent with incomes under 133 percent of poverty ($14,484 for a single person) had a gap in coverage in 2011, more than three times the rate of those with incomes over 400 percent of poverty ($43,560 for a single person).

Only 17 percent of young adults ages 19 to 25 in low-income families stayed on or joined their parents' plans, compared with 69 percent of young adults in the highest income households. Young adults over 25 cannot take advantage of the early provision.

Young adults delay care regardless of being insured

Whether or not they had health insurance, many young adults skipped or delayed getting needed health care because of cost, The Commonwealth Fund reports. Two in five said they did not fill a prescription; skipped a medical test, treatment or follow-up visit recommended by a doctor; did not go to a doctor when sick; or did not get needed specialist care.

Of course the statistics are worse for the uninsured. Fifty-six percent of those who had an insurance gap during the year did not get needed care because of cost, compared with 29 percent of young adults who were insured all year.

Going without health insurance also meant young adults were not in contact regularly with a doctor or place of care. Seventy-two percent with gaps in coverage and 38 percent without insurance at the time of the survey didn't have routine check-ins with doctors. Similarly, rates of preventive care, including weight and blood pressure checks, were lower for those with coverage gaps and continued to drop for young adults without insurance.

Timely use of dental care also varied depending on whether young adults had insurance. Young adults who were continuously insured under a plan that included dental coverage had the highest rates of dental care: three-quarters (76%) had had a dental check-up in the past year. Only 54 percent of those who had a gap in their insurance coverage in the past year and just 28 percent of those who were uninsured at the time of the survey had had a dental exam.