There is limited data on long-term outcome of ASD with co-occurring intellectual disabilities (ID) and challenging behaviours in France. The EpiTED period cohort is a 15 years longitudinal study of the developmental trajectories of 281 children initially recruited at mean age of 5 years. Two contrasted developmental trajectories were identified. Low cognitive level, absence of language, and higher ASD scores at baseline were predictive of low growth at follow-up. As adults the participants were predisposed to persistent co-occurring challenging behaviours as well as underlying ID impacting their ability to function independently. The results underscore the need for development of services and supports for adults with ASD in France who may also have already lacked access to adequate interventions and support services.