BACKGROUND:: Little is known about the time-varying determinants of kidney graft failure in children. METHODS:: We performed a retrospective study of primary pediatric kidney transplant recipients (<18 years) from the Eurotransplant registry (1990-2020). Piece-wise exponential additive mixed models were applied to analyze time-varying recipient, donor, and transplant risk factors. Primary outcome was death-censored graft failure. RESULTS:: We report on 4528 kidney transplantations, of which 68% with deceased and 32% with living donor. 1638 recipients experienced graft failure, and 168 died with a functioning graft. Between 2011-2020, the 5-year graft failure risk was 10% for deceased donor and 4% for living donor kidney transplant recipients. Risk of graft failure decreased 5-fold from 1990 to 2020. The association between living donor transplantation and the lower risk of graft failure was strongest in the first month posttransplant (aHR:0.58, 95%CI:0.46-0.73) and remained statistically significant until 12 years posttransplant. Risk factors for graft failure in the first 2 years were deceased donor age <12 or >46 years, potentially recurrent kidney disease, and PRA>0%. Other determinants of graft failure included dialysis prior to transplantation (until 5 years posttransplant), HLA-mismatch 2-4 (0-15 years posttransplant), HLA-mismatch 5-6 (2-12 years posttransplant), and hemodialysis (8-14 years posttransplant). Recipients >11 years at transplantation had a higher risk of graft failure 1-8 years posttransplant compared to other age groups, whereas young recipients had a lower risk throughout follow-up. Analysis of the combined effect of posttransplant time and recipient age showed a higher rate of graft failure during the first 5 years posttransplant in adolescents compared with young transplant recipients. In contrast to deceased donors <12 years, deceased donor age >46 years was consistently associated with a higher graft failure risk. CONCLUSIONS:: We report a long-term inverse association between living donor kidney transplantation and the risk of graft failure. The determinants of graft failure varied with time. There was a significant cumulative effect of adolescence and time posttransplant. The “ideal donor age window” was dependent on time posttransplant.