This study aimed to: (1) investigate the prevalence and magnitude of the Relative Age Effect (RAE) across a national team program; (2) assess the transition rates from youth to senior international level, and (3) examine the influence of birth quartile distribution on transition. 1518 male and 487 female soccer players from a national soccer association were examined. All participants were divided into birth quartiles: January–March (BQ1), April–June (BQ2), July–September (BQ3), and October–December (BQ4). Significant RAE were observed across all age groups, with small-to-medium effect sizes (Cramer’s V: 0.10–0.19), except for male U-16, female U-16, and U-19 players. Senior teams exhibited the most significant RAE, with odds ratios of 2.27 for male and 1.50 for female players. Transition rates from youth to senior teams were higher for female (30%) than male (20%) players. Contrasting trends by birth quartile were evident with males having the lowest transition rate in BQ1 (17%), while females had the lowest transition rate in BQ4 (25%). The number of youth team appearances in both sexes was a significant predictor of transition probability (β = 0.177), with each additional appearance increasing the likelihood of progressing to the senior team by 19.4%. A significant interaction between sex and youth team appearances indicated that the impact of playing opportunities was less pronounced for male players than females. These findings highlight the influence of the RAE in talent development and underscore the need for equitable opportunities for all players, regardless of sex.