The Affordable Care Act allowed young adults to enroll as dependents on their parents' private health insurance plans until age 26. This study found that the policy change was associated with a 5.3 percentage point increase in private dependent coverage and a 3.5 percentage point decrease in uninsured rates among young adults targeted by the federal policy alone. For those also targeted by prior state dependent coverage laws, the effects were even greater, with an 8.7 percentage point rise in private dependent coverage. The results suggest the federal policy had a significant positive impact on insurance coverage for young adults.