Background: Pediatric leukemia exhibits marked variation across developmental stages and between sexes. While lymphoid predominance in early childhood is well established, agedependent shifts in subtype distribution and sex-modified effects remain undercharacterized in middle-income settings. We evaluated developmental heterogeneity and age–sex interaction patterns using national population-based data. Methods: All pediatric leukemia cases (0–19 years) recorded in a national cancer registry were pooled (n = 2,829). Cases were stratified into age groups (0–4, 5–9, 10–14, 15–19 years) and categorized by lineage subtype. Subtype proportions were calculated within age strata. Logistic regression assessed odds of acute myeloid leukemia (AML) relative to acute lymphoblastic leukemia (ALL), using ages 0–4 years as reference. Age × sex interaction was evaluated using chi-square testing. Results: Early childhood (0–4 years) accounted for 39.0% of cases. Lymphoid leukemia predominated in children aged 0–4 (71.2%) and 5–9 years (75.2%) but declined to 49.3% in adolescents. AML increased from 13.8% in early childhood to 34.2% in adolescents, with the AML:ALL ratio rising from 0.19 to 0.69. Adolescents demonstrated 3.65-fold higher odds of AML compared with children aged 0–4 years (95% CI 2.70–4.93; p < 0.001). Male predominance was observed across all age groups, and a significant age × sex interaction was identified (? ² =192.86; p = 1.53 × 10? ²? ). Conclusions: Pediatric leukemia demonstrates a robust developmental gradient with significant sex-modified subtype heterogeneity . The adolescent expansion of AML suggests age-dependent lineage susceptibility and supports a developmental framework for leukemogenesis.