Objective This study aimed to investigate whether active outdoor play and/or sports at age 10 is associated with sport/physical activity at 32 year follow-up using a birth cohort study. Methods Data were from the 1970 British Cohort Study, a longitudinal observational study. The present paper included data from the age 10 year and age 42 year surveys. At age 10 the participant’s mother provided information regarding how often their child played sports, and played outside on streets, parks or playgrounds. At age 42 participants reported frequency of participation in physical activities and sports. Associations between participation in sport/active outdoor play at aged 10 years and adult sport/physical activity were investigated using adjusted (gender, fathers socio-occupational class, child’s BMI, father’s BMI, self-rated health at age 42, assessment of own weight at age 42, participant’s education) Cox regression. Results Final adjusted Cox regression models showed that participants (n = 6,458) who often participated in sports at age 10 were significantly more likely to participate in sport/physical activity at age 42 (RR 1.10; 95% CI 1.01 to 1.19). Active outdoor play at age 10 was not associated with participation in sport/physical activity at age 42 (RR 0.99; 95% CI 0.91 to 1.07). Conclusion Childhood activity interventions might best achieve lasting change by promoting engagement in sport rather than active outdoor play.