Background: We sought to assess the association between age and the prevalence of obesity among children with and without autism spectrum disorder (ASD) in the 2011-2012 National Survey of Children's Health. Methods: Analyses were restricted to 43,777 children, ages 10-17, with valid measures of parent-reported weight, height, and ASD status. Exploratory analyses describe the impact of sex, race/ethnicity, and household income on the relationship between age and obesity in ASD. Results: Although the overall prevalence of obesity among children with ASD was significantly (p<0.001) higher than among children without ASD (23.1% vs. 14.1%, 95% confidence interval for difference 3.6 to 14.4), child age significantly (p=0.035) modified this difference. In a multivariable logistic regression analysis, adjusted for sex, race/ethnicity, and household income, the odds of obesity among children with ASD compared with children without ASD increased monotonically from ages 10 to 17 years. This pattern arose due to a consistently high prevalence of obesity among children with ASD and a decline in prevalence with advancing age among children without ASD. These findings were replicated using a propensity score analysis. Exploratory analyses suggested that the age-related change in obesity disparity between children with and without ASD may be further modified by sex, race/ethnicity, and household income. Conclusions: The patterns of prevalence observed with increasing age among children with and without ASD were unexpected. A better understanding of the etiological and maintenance factors for obesity in youth with ASD is needed to develop interventions tailored to the specific needs of these children.