Purpose: Developmental changes in sleep in youth with autism spectrum disorder (ASD) are understudied. In non-ASD youth, adolescents exhibit a "night owl chronotype" (i.e., later sleep/wake timing) and social jetlag (i.e., shifts in sleep timing across school nights and weekends), with corresponding sleep problems. The purpose of this study is to evaluate age trends in chronotype, social jetlag, and sleep problems in high-risk youth with ASD. Methods: Youth with ASD (N = 171), ages 5-21 years old, were enrolled at the time of admission to specialized psychiatric units. Caregivers reported children's demographic information, habitual sleep timing, and sleep problems. Multivariate analyses evaluated the effect of age on chronotype, social jetlag, and sleep problems and the effects of chronotype and social jetlag on sleep problems. Covariates and moderators included sex, race, verbal ability, autism symptom severity, supplemental melatonin, and pubertal status. Results: Older age was associated with later chronotype, more social jetlag, fewer sleep anxiety/co-sleeping problems, fewer night waking and parasomnia problems, and more daytime alertness problems. The effect of age on chronotype was stronger for youth with greater social affective symptom severity. Mediation analyses showed that later chronotype statistically mediated the association between age and daytime alertness problems. Conclusions: Youth with ASD may exhibit night owl chronotype behavior and social jetlag as they enter adolescence. Shifts toward a later chronotype may be exacerbated by autism severity and may contribute to alertness problems and sleepiness during the day. Chronotype is modifiable and may be leveraged to improve daytime functioning in youth with ASD.