Temporal trends in the relative age effect in five major European men's football leagues, 2000-2025

The relative age effect (RAE) - the overrepresentation of athletes born early in the selection year - has been widely documented across youth and professional sport. However, less is known about how birth-quarter distributions among elite professional footballers have changed over extended historical periods. This descriptive-longitudinal study examined birth-quarter and birth-month distributions among male players in the Premier League, La Liga, Bundesliga, Serie A, and Ligue 1 across 25 seasons, from 2000-2001 to 2024-2025. The final dataset comprised 60,078 player-season records representing 14,173 unique players who accumulated at least 90 minutes of league playing time. Across the pooled sample, birth-quarter distributions differed significantly from a uniform expectation, with Q1-born players more frequently represented than Q4-born players (30.4% vs. 20.1%; χ²(3) = 1292.0, p < .001; Cramér's V = 0.085). The same pattern was observed in the unique-player analysis (V = 0.085), suggesting that the pooled distribution was not driven solely by repeated seasonal observations of the same players. Season-level estimates increased during the earlier part of the observation window and subsequently stabilised, rather than showing continuous linear amplification across the full 25-season period. This temporal pattern varied substantially by league and requires interpretation in light of cohort replacement, player mobility, developmental-cutoff uncertainty, and changing league composition.