Objective To estimate health care expenditure trends among young adults ages 19–25 before and after the 2010 implementation of the Affordable Care Act (ACA) provision that extended eligibility for dependent private health insurance coverage. Data Sources Nationally representative Medical Expenditure Panel Survey data from 2008 to 2012. Study Design We conducted repeated cross‐sectional analyses and employed a difference‐in‐differences quantile regression model to estimate health care expenditure trends among young adults ages 19–25 (the treatment group) and ages 27–29 (the control group). Principal Findings Our results show that the treatment group had 14 percent lower overall health care expenditures and 21 percent lower out‐of‐pocket payments compared with the control group in 2011–2012. The overall reduction in health care expenditures among young adults ages 19–25 in years 2011–2012 was more significant at the higher end of the health care expenditure distribution. Young adults ages 19–25 had significantly higher emergency department costs at the 10th percentile in 2011–2012. Differences in the trends of costs of private health insurance and doctor visits are not statistically significant. Conclusions Increased health insurance enrollment as a consequence of the ACA provision for dependent coverage has successfully reduced spending and catastrophic expenditures, providing financial protections for young adults.