Sex differences have been found amongst toddlers and young children with autism spectrum disorder (ASD). We investigated the presence and stability of these ASD sex differences throughout childhood and adolescence. Participants (N = 325, 52 females; aged 3–18 years) consecutively received an ASD diagnosis at a clinic for assessing high-functioning ASD (mean verbal IQ = 92.6). There were no IQ sex differences. By parent report and direct observation, females had less repetitive stereotyped behaviour (RSB), with male-equivalent levels of social and communication impairment. Teachers reported males with ASD as having greater externalising and social problems than females. The female phenotype we describe was stable across our sample’s age range. Their milder RSBs and less severe difficulties at school may lead to under-recognition of ASD in females.