Previous studies have reported reduced quality of life in autism. Improving quality of life for autistic people is, therefore, a key priority for clinical research and practice. However, the relative impact of core autism traits (e.g. social-communication difficulties), as compared to associated mental health symptoms (e.g. anxiety, depression) on quality of life remains poorly understood. This is despite at least 20%-50% of autistic individuals experiencing associated anxiety and/or depression symptoms. Hence, we measured subjective quality of life in 573 six to thirty-year-olds (autism spectrum disorder N=344), using two widely validated questionnaires. Adults self-reported on the World Health Organization Quality of Life-Brief instrument. Parents of children/adolescents completed the Child Health and Illness Profile. We assessed individual variability across both measures and modelled associations between quality of life, core autism traits, anxiety, and depression symptoms. Across both age groups and quality of life measures, autistic individuals scored lower than comparison individuals, on average, particularly for physical health in adults (d=-1.24, 95% confidence interval: [-1.56, -0.93]) and school achievement for children/adolescents (d=-1.06, 95% confidence interval: [-1.29, -0.84]). However, a notable proportion of autistic individuals (36%--71% across quality of life domains) did not have reduced quality of life. Across ages and quality of life measures, severity of associated symptoms was significantly related to reduced quality of life on several domains, after accounting for core autism traits. Most notably, depression symptoms were related to reduced physical/psychological well-being in both adults ([beta][greater than or equal to]-0.34) and children/ adolescents ([beta]=-0.29, 95% confidence interval: [-0.36, -0.14]). For children/adolescents, anxiety symptoms ([beta][greater than or equal to]-0.28) and core social-communication difficulties ([beta][greater than or equal to]-0.22) were also related to subjective quality of life outcomes. Overall, findings indicate that not all autistic individuals experience reduced subjective quality of life. Variability in quality of life is significantly influenced by associated symptoms, across developmental stage. This may provide a tractable target for mental health services to improve quality of life for autistic individuals over the lifespan.