Previous research has indicated that autistic adults experience higher rates of co-occurring mental health difficulties and poorer quality of life (QoL) than their non-autistic peers. Little is known, however, about these aspects in older age or whether younger and older autistic adults experience similar patterns This cross-sectional study investigated potential age-related effects on autism symptoms, self-reported mental health, and QoL in younger and older autistic adults (n = 79, aged 19-71 years) compared to a non-autistic control group (n = 57) matched for gender, age and IQ. Results showed that autistic adults had higher levels of self-reported autism symptoms and poorer QoL than controls. There were no significant age effects on autism symptoms or on most self-rated mental health symptoms. However, significantly more autistic adults in the younger versus older group scored above the clinical threshold for anxiety, somatoform disorders and eating disorders. Older autistic adults rated social QoL as significantly better than younger autistic adults; there was no significant age difference in the control group. Self-reported QoL was best predicted by self-ratings of severity of depressive symptoms in both groups. Further research is needed to track autism and co-occurring mental health symptomatology across the lifespan, so that service provision can be tailored accordingly. LAY SUMMARY: Young autistic adults have reported more psychological difficulties and poorer quality of life (QoL) than the general population. We investigated whether these difficulties continue into older age. Autism symptoms and mental health problems were common in autistic adults, with no difference between age groups, except for anxiety, physical and eating problems. Although QoL was poorer in both younger and older autistic compared to non-autistic adults, older autistic adults reported better social QoL than those who were younger.