Longitudinal records were examined for 272 children who started formal swimming lessons at 3, 4, 5, 6, 7, or 8 years of age. The groups of children were compared on the number of lessons and number of months required to reach three levels of swimming proficiency and the ages at which these levels were reached. Significant differences were found in the ages at which each proficiency level was attained. The younger the age at which children started lessons, the younger they were when they attained Level 1 proficiency. In contrast, the ages at which Level 2 and Level 3 proficiency were attained were more similar, particularly for the younger starters. Significant differences were found for the number of lessons and number of months required to reach Level 1 proficiency and to progress from Level 1 to Level 2 proficiency. The younger the starting age the more lessons and months required to reach each proficiency level. The pattern of findings suggests that the optimum age to begin formal swimming lessons is between 5 and 7 years of age. The findings are discussed relative to the literature on readiness and sensitive periods for learning.