Sara Collins, vice president of the health care coverage and access program at the Commonwealth Fund, agreed that the increase in those staying on their parents' plans is a hopeful sign.
"This adds more evidence to prior research of how successful this provision has been in the Affordable Care Act, allowing young adults to join their parents' plans," Collins said.
"This is the first time we have seen a decline in number of uninsured young adults. This provision has exceeded expectations," she added.
Stoll said that as more young adults age out of eligibility for their parents' plans, they are likely to get their own plans through health marketplaces such as HealthCare.gov.
Moving forward, many young adults will be eligible for financial assistance in the form of a tax cut to help them buy a private health insurance plan, she noted.
"That tax credit will be quite robust for many young adults, who often don't have a lot of income as they launch their careers," Stoll said.
Some young adults don't see the need to pay for health insurance, because they think they're "invincible," Stoll pointed out.
"But many young adults have experienced, in their own life or among their peers, how quickly health can deteriorate. A sudden illness or unexpected injury can leave you vulnerable, not only physically but financially. It can put you back for years. Medical debt can really change a young person's options," she said.
Young adults do have to be educated about the benefits of being insured and the tax cuts that will make it affordable, Stoll said.