Dropped Out Before They Ever Competed: A Population-Wide Registry Analysis of Relative Age Effects in Norwegian Track and Field from Age 10 to 25

Background. Childhood participation in organised sport is one of the strongest predictors of adult physical activity, and the relative age effect (RAE) has recently been linked to lower physical activity and higher sedentary behaviour among schoolchildren. The dominant theoretical account locates RAE generation in adolescent talent-selection cycles, predicting amplification through adolescence. If selection instead occurs before children enter organised sport, the health consequences require intervention at the entry stage rather than at adolescent talent identification. We tested this prediction using population-wide registry data 2011–2026 from the Norwegian Athletics Federation (87 624 athletes; 1 395 788 performances; the entire competing cohort), comparing birth-quarter distributions across ages 10–25 against monthly birth counts from Statistics Norway. Logistic regression with sex age interaction tested for adolescent amplification; Manski-style bounds quantified the influence of athletes appearing briefly without a recorded birth date (61 % at age 10–12; the earliest dropouts). Results. At age 10 — the youngest registered competition age — Q1 comprised 28.3 % and Q4 19.7 % of competitors, against population baselines of 24.6 % and 22.9 % (χ 2 =76.0, p<10 (-15) , Cohen’s w=0.10 ). The Q1/Q4 ratio of 1.44 was indistinguishable at every age through 25 (range 1.32–1.56). Logistic regression did not support the cumulative-advantage prediction in females (β age =-0.007, p=0.46), showing only a mild curvilinear elevation in males peaking at age 18–19 (p=1.0×10 -5 ) for the sex x age interaction). First-year retention from age 10 was nearly identical across quarters (Q1–Q4 gap 2–3 pp), confirming the skew was not produced by differential dropout within the registry. Findings were robust to exclusion of the COVID-19 pandemic years. Conclusions. In a population-wide individual sport without team selection, RAE is established by age 10 and stable through senior level. This pattern is inconsistent with the cumulative-advantage prediction and points to pre-entry selection. We hypothesise that mastery experience through measurable personal progress retains surviving athletes — a feature absent from team-sport contexts. Public-health interventions should be tested at the sport-entry transition, not only at adolescent talent-identification stages.