Pediatric sepsis is a leading cause of mortality in children across the world. Within pediatric sepsis, how developmental age-specific host immune response impact on the occurrence and development of pediatric sepsis is still unknown, especially between infants and toddlers, which were the major susceptible age groups in sepsis. Experimental design In this study, we applied a nested case-control study strategy and analyzed the plasma proteomes of pediatric sepsis patients between infants and toddlers in comparison to their age-matched controls respectively. Each age group consists of three subgroups with different outcomes. One hundred and ten plasma samples were pooled into 16 samples for quantitative identification by LC-MS/MS. Results It was totally quantified 677 proteins. In comparison to toddlers, infant sepsis patients were characteristic with dominant neutrophil-mediated defense, suppressed adaptive immunity and NK-mediated cytotoxicity. Besides, pentose phosphate pathway was more up-regulated in infants and associated with poor outcome. Moreover, combined Hp and Thbs1 with the AUC value of 0.958 (95%CI, 0.868, 1.000) was confirmed as potential infant-adapted prognostic marker of poor outcome in sepsis. Conclusion and clinical relevance Our proteomic analysis of age-associated pediatric sepsis combined with clinical laboratory data provided a comprehensive insight into the complex, multifactorial heterogeneous host response to pediatric sepsis and allowed identification of critical differences between infant and toddler sepsis. Moreover, we confirmed that combined Hp and Thbs1 is more infant-adapted and serves as a potential prognostic biomarker for poor outcome of infant sepsis, promoting the application of age-adapted precision medicine in pediatric sepsis.