Purpose <p>To determine the prevalence and causes of reduced vision in childhood.</p> Methods <p>Cross-sectional analysis included five cohorts aged; 3–5 years old (<i>n</i> = 986) from the Sydney Paediatric Eye Disease Study (SPEDS), 6 years (<i>n</i> = 1739) and 12 years (<i>n</i> = 2345) from the Sydney Myopia Study (SMS) and the 5 year follow-up of SMS at 12 years (<i>n</i> = 1111) and 17 years (<i>n</i> = 1649) in the Sydney Adolescent and Vascular Eye disease Study (SAVES). All children had a comprehensive ocular examination, including visual acuity (VA) with and without refractive correction. Reduced uncorrected VA was defined as worse than 6/12 (logMAR 0.30). A longitudinal analysis was also conducted to identify incident cases of reduced vision between SMS baseline and SAVES follow-up among children assessed at both time points.</p> Results <p>The overall prevalence of reduced vision based on unaided visual acuity increased with age from 4.5% in SPEDS 3–5 year olds to 17.7% in SAVES 17 year olds (<i>p</i> &lt; 0.001). The proportion of reduced vision caused by refractive error, amblyopia, strabismus and pathology also varied between cohorts. In the younger cohorts, amblyopia accounted for 21% of SPEDS 3–5 year olds and 28% of SMS 6 year olds with reduced vision. There was a substantial decline in the proportion of reduced vision attributed to amblyopia in the older cohorts, while refractive error increased. Myopia contributed to 18% of children with reduced vision in SPEDS 3–5 years and SMS 6-year-olds and increased to over 80% in SMS 12-year-olds, SAVES 12-year-olds and 17-year-olds.</p> Conclusions <p>Targeted preschool screening for amblyopia is supported as it occupied a higher proportion of&#xa0;the younger ages. Community education to promote wearing glasses in adolescence for those with myopia is warranted.</p>