Under health care reform, adult children are allowed to stay on their parent's health insurance plan until the age of 26. The Employee Benefit Research Institute says that previous estimates of between 680,000 to 2.1 million young adults taking advantage may be incorrect.

The EBRI notes that it's not easy to estimate how many parents will decide to enroll their adult children onto their health insurance plan. They note that some individuals within the 19-25 year old age bracket that currently get their health insurance through another medium, such as Medicaid, may switch to their parent's plan.

"It is critical that group plans and insurers understand the size and characteristics of the 19 to 25-year-old population that might be eligible for their parents' health coverage in order to determine the impact that this provision of PPACA may have on enrollment and costs of employment-based coverage," wrote EBRI health research and education program director Paul Fronstin.

Young adults are are more likely than any other age group to go without health insurance. Government data shows that 39 percent of adults between the ages of 19 and 29 lack health insurance.