BACKGROUND: Uninsurance for young adults (YAs) was greatly reduced by the Affordable Care Act (ACA). However, reforms may not be equally beneficial for all YAs and certain policies may exacerbate, rather than resolve, pre-existing disparities. OBJECTIVE: To investigate inequalities in YAs' insurance coverage pre- (2000-2010) and post-federal health reforms (dependent coverage expansion, 2010-2013, and Medicaid/Marketplace expansions, 2014-2016), among a nationally representative sample. DESIGN: A difference-in-differences estimator (controlling for sociodemographics) was used to determine the effects of the ACA for young adults (ages 19-25) compared to adolescents (ages 13-18) and older YA (ages 26-30) counterparts; triple-difference estimators quantified differential policy effects by sociodemographics. PARTICIPANTS: Three hundred eighty-seven thousand six hundred thirty-five participants in the 2000-2016 National Health Interview Survey. MAIN MEASURES: Respondents reported their health insurance coverage types during the last 12 months, reasons uninsured, and detailed sociodemographics. KEY RESULTS: An adjusted difference-in-differences estimator quantified a 12.3 percentage point increase (p < 0.0001) in full-year coverage post-ACA for YAs compared to older counterparts, driven by increases in employer-sponsored private insurance while younger and older youth saw larger gains in Medicaid coverage. Triple-difference estimators identified subgroups experiencing less beneficial dependent coverage expansion effects, including females, lower socioeconomic status, non-citizens, non-English speakers, and several racial/ethnic minority groups. Later ACA reforms (Marketplace/Medicaid expansions) mitigated many of these widening disparities. CONCLUSION: While the ACA significantly impacted YA insurance coverage, these gains were not of equal magnitude for all YAs and disparities remain. As such, more work needs to be done to ensure optimal and equitable access to high-quality, affordable insurance for all YAs.