--- - |2+ Social evaluative threat is a potent activator of the hypothalamic‐pituitary‐adrenocortical (HPA) axis in typically developing (TD) populations. Studies have shown that children and adolescents with autism spectrum disorder (ASD) show a blunted cortisol response to this type of stressor; yet, a previous study in adults with ASD reported a more prototypical stress response. The current study compared 24 adolescents and 17 adults with ASD to investigate a possible developmental lag in autism resulting in a more adaptive stress response to social evaluation with development. Participants were exposed to the trier social stress test (TSST), and salivary cortisol was collected before and after stress induction. Multilevel modeling revealed that relative to adolescents, young adults with ASD evidenced a significant increase in cortisol in response to anticipatory stress, and 23.5% were classified as anticipatory responders. Adolescents, however, had a significant change in slope in response to the TSST, with 37.5% classified as reactive responders. In both groups, the majority of participants did not have a robust stress response to the TSST as would be expected in TD participants. Findings suggest significant differences in the cortisol trajectory; adults with ASD were more likely to show an anticipatory response to being socially evaluated, which was maintained throughout the stressor, whereas the adolescents had a more reactive response pattern with no anticipatory response. Further research is needed to determine if such patterns are adaptive or deleterious, and to determine underlying factors that may contribute to distinct stress profiles and to the overall diminished stress responses. Lay Summary Many individuals have increased stress when being socially evaluated. The current study shows that adults with ASD have increased stress in anticipation of a task in which individuals are required to give a speech to unfamiliar raters, while adolescents with ASD tend to show a stress response only during the task itself. Further research is necessary to understand whether developmental influences on stress response in ASD have significant impacts on other areas of functioning often affected by stress. - Autism Research, EarlyView.