Prior to implementation of the Patient Protection and Affordable Care Act, dependent health insurance coverage was typically available only for young adults under the age of 19. As of September 2010, the Affordable Care Act extended dependent health insurance coverage to include young adults up to the age of 26. I use the National Health Interview Survey for the sample period from 2011 to 2013 to analyze the causal relationship between the expansion of dependent coverage and risky behaviors including smoking and drinking as well as preventive care. I employ a regression discontinuity design to estimate the causal effect of health insurance coverage and overcome the endogeneity problem between insurance status and risky behaviors. When young adults become 26 years old, they are 7 to 10 percentage points more likely to lose health insurance than young adults under the age of 26. Although young adults over the age of 26 are generally aged out of insurance coverage, presence or absence of health insurance does not affect their smoking and drinking behaviors and their access to preventive care.