Repeated sprint exercise (RSE), widely used in intermittent sports, induces immune changes critical to address for optimizing training and reducing health risks in youth athletes, especially across biological maturation (BM) stages. We analyzed RSE effects on immune factors in adolescent and adult athletes, considering BM stages. Twenty-nine male intermittent-sport athletes (19 hebiatric: 10 pre-peak height velocity [PHV] [12.1 ± 0.6 years], 9 circum-PHV [13.8 ± 0.7 years]; 10 adults [23.2 ± 2.1 years]) performed RSE (3 × 6 × 35 m sprints, 10-sec rest, 5 min interset). Blood was collected pre-, post-, 2 h, and 24 h post-RSE to assess lactate, leukocyte subsets (neutrophils, monocytes, lymphocytes, T/B cells, NK-cells), and cytokines. According to international guidelines for athletic classification, 72% of the participants in this study were classified as level 2 (regional; ~12-19% of the global population), 17% as level 3 (national; ~0.014% of the global population), and 11% as level 4 (international; ~0.0025% of the global population). RSE elevated leukocytes in all groups, with adults showing higher neutrophils and hebiatric athletes elevated T/B cells (p < 0.05). Pre-PHV exhibited reduced neutrophil/cytokine responses versus circum-PHV/adults (p < 0.05). Circum-PHV displayed post-RSE CD4+/CD8 + rises, while adults had the lowest CD4 (p < 0.05). Pre-PHV peaked in NK/B-cells at 2 h/24 h; adults showed elevated IL-6/IL-8 (p < 0.05). All parameters normalized by 24 h. Immune responses to RSE differ by BM stage, with adults exhibiting heightened inflammation. Tailoring training to BM stages may optimize performance and reduce immunosuppression risks, particularly in hebiatric athletes.