Purpose To investigate the age range for which cycloplegia provides additional information compared with non-cycloplegic refraction in teenagers and young adults. Methods Data for 1295 subjects (704 female; 591 male) from the Twins Eye Study in Tasmania (TEST) and the Brisbane Adolescent Twin Study (mean age: 19.65 ± 3.56, range: 13-26 years) were included. For all participants, cycloplegia was induced by instillation of either one drop of 1% cyclopentolate (13-14 years) or one drop of 1% tropicamide (15-26 years). Pre- and postcycloplegic refractive errors for both eyes were measured using a Humphrey-598 automated refractor and spherical equivalents of refractive error were calculated. Generalized Estimating Equations (GEE) were used to model the spherical equivalent refraction (SER) for each eye against age (by year) and axial length (in the given eye). Results The mean group difference between pre- and postcycloplegic SER (post minus pre) was 0.17 ± 0.52 D and 0.12 ± 0.51 D for the right and left eyes, respectively, indicating that postcycloplegic refraction was generally more hyperopic/less myopic. The mean difference between pre- and postcycloplegic SER decreased from 0.36 ± 0.41 D in the 13-year-olds to 0.06 ± 0.50 D in people aged 25 years. After adjusting for family-relatedness, the difference between pre- and postcycloplegia SER was significant in all age groups up until the age of 20 years. Conclusions Non-cycloplegic autorefraction can result in group mean SER differences of greater myopia than cycloplegic autorefraction and occurs in teenagers (13-19 years of age), but not in adults 20-26 years. These data suggest that cycloplegia is not required in population estimates of refractive error for young adults once they reach approximately 20 years of age.