That key provision ensures that those aged 19 to 25 have the option of remaining on their parents' health insurance plan. It's been in effect since September 2010.
In the new study, researchers found that the move may be allowing more young adults to receive mental health care. Soon after the rule took hold, the number of 18- to 25-year-olds in mental health treatment rose nationally -- by a couple of percentage points among those who had potential symptoms of psychiatric conditions.
"That's very good news," said Dr. Jeffrey Borenstein, a psychiatrist and president of the Brain & Behavior Research Foundation in New York City.
"Often, mental health disorders like schizophrenia, depression and substance abuse first manifest in this age group," said Borenstein, who was not involved in the study. At the same time, he added, those disorders can keep some young adults from holding a job -- and gaining their own health insurance.
Brendan Saloner, one of the researchers on the study, agreed that young adults are "particularly vulnerable" for those reasons.
Based on the new findings, Saloner said, the rule seems to be a "stepping stone" to improving young Americans' access to mental health care.
Saloner, an assistant professor at the Johns Hopkins Bloomberg School of Public Health in Baltimore, was a Robert Wood Johnson Foundation scholar at the time of the study. He and colleague Benjamin Le Cook report the findings in the August issue of the journal Health Affairs.
For the study, the researchers used data from an annual government health survey that included questions used to screen for mental health or substance abuse problems. Between 2008 and 2012, there were over 20,000 18- to 25-year-olds with potential signs of those conditions.
Saloner's team found that before the rule took effect, about 31 percent of those young people had received some type of mental health treatment in the past year. After the rule went into effect, that percentage rose to 33 percent.
The increase was small, but looked bigger when compared with the trend among 26- to 35-year-olds, whose mental health treatment rate actually dipped over the study period.
What's more, Saloner said, there was an increase in the number of young people who said private insurance paid for their mental health care. After the rule, 33 percent said private insurance paid for their last treatment, versus 29 percent in the two years before the rule went into effect.
Meanwhile, the reverse was true for 26- to 35-year-olds. In 2008-2009, over 38 percent said private insurance covered their last mental health treatment. After 2010, that dropped to 29 percent.
"We do see an effect of the [Affordable Care Act] on financial protection for young adults," Saloner said.
Still, both he and Borenstein said insurance coverage is just one part of getting mental health treatment to the young adults who need it.
For one, people need providers in their community who can give them the type of treatment they want. "There are areas of the country that are underserved," Saloner said. "Just because you have insurance doesn't mean you'll have access to a provider, or to one who can see you in a reasonable amount of time."
According to Borenstein, another barrier is the continuing stigma attached to mental health conditions.
"There is prejudice and discrimination against people with mental health disorders, so some shy away from treatment," he said. "We need to change that. People shouldn't suffer in silence."
The U.S. Department of Health and Human Services has more on young people's mental health.