To identify if age influenced the relationship between one of the central symptoms of Autism Spectrum Disorder (ASD) and physiological stress, the association between stereotypic behaviour (SB) and stress-related cortisol concentrations was examined in a sample of 150 young males with an ASD. Parent-rated SB was significantly correlated with cortisol concentrations for boys aged 6 years to 12 years but not for adolescents aged 13 years to 18 years. This age-related difference in this association was not a function of cortisol concentrations but was related to differences in SB across these two age groups. IQ did not have a significant effect on this relationship, suggesting that age-related learning may have been a possible pathway for reduced SB during adolescence. The aspect of SB that was most powerfully related to cortisol was general repetitive behaviour rather than movements of specific body parts. Explanations of these findings are raised for further investigation.