BackgroundAutism and autistic traits onset in childhood but persist into adulthood. Little is known about how genetic and environmental factors influence autism and autistic traits into adulthood. We aimed to determine age effects on the heritability of clinically diagnosed autism and the etiological stability of autistic traits from childhood to adulthood using twin methods.MethodsFrom 23,849 twin pairs in the Swedish Twin Register born between 1959 and 2010, we identified 485 individuals (1.01%, 31.5% female) with a clinical autism diagnosis. We estimated and compared the relative contribution of genetic, shared, and nonshared environmental influences to autism in childhood and adulthood. We further used multivariate twin analysis with four measurement points among 1,348 twin pairs in the longitudinal Twin Study of Child and Adolescent Development to assess the phenotypic and etiological stability of autistic traits – measured with three scales from the Child Behavior Checklist – from childhood to adulthood.ResultsAutism heritability was comparable from childhood, (96% [95% CI, 76–99%]) to adulthood (87% [67–96%]). Autistic traits were moderately stable (phenotypic correlation = 0.35–0.61) from childhood to adulthood, and their heritability varied between 52 and 71%. We observed stable as well as newly emerging genetic influences on autistic traits from ages 8–9 to 19–20, and unique nonshared environmental influences at each age.ConclusionsGenetic factors are important for autism and autistic traits in adulthood and separate genetic studies in adults are warranted.