We explored the hypotheses that adolescents have more negative obesity stereotypes than children and that age differences in obesity stereotypes are mediated by weight attributions, body esteem, and appearance idealization. Hispanic- and Caucasian-American children completed measures of appearance idealization, body esteem, and attributions about the causes of obesity. Participants later made generalizations from and attributions to "target" children, whose pictures varied by weight, gender, and ethnicity. Despite modest variations by participant ethnicity, age-related increases in obesity biases were (a) found across both stereotyping measures, (b) evident for both Hispanic and Caucasian targets, and (c) most pronounced for female targets. Although the association between age and obesity biases was partially mediated by appearance idealization and body esteem, these variables did not entirely explain the observed age-related increases in biases. Discussion focuses on the psychosocial atmosphere of adolescence, a "thin is in/fat is out" heuristic, and the development of implicit and explicit processing systems. (Contains 5 tables and 1 figure.)