Background: Kidney transplantation is the best treatment for end stage kidney disease. The allograft allocation system considers immunological factors and time in waiting list, but not the age of donors or recipients, in spite that both have prognostic relevance. Aim: To study the fairness of the allograft allocation system considering the age of donors and recipients. Material and methods: Based in the age distribution of donors and waiting list of patients for kidney transplantation, we constructed probability functions for receiving a graft as an allocation system based on age changes from a strict to a more lax criteria. Finally, we compared our model with the real transplants performed during 2012. Results: The probability distribution for receiving a kidney transplantation is displaced to the right compared with the age distribution of graft donors. This gap increases as the procuring hospitals enlist progressively older graft candidates without accepting older donors in parallel. An allocation system that maintains age parity between donors and recipients is fairer that other that allows lax criteria for the same allocation. This phenomenon is attenuated if the procurement acceptance criterion allows older donors. Conclusions: It is necessary to reduce the gap between donor and recipient ages for kidney transplantation to improve the fairness of the graft allocation system.Background: Kidney transplantation is the best treatment for end stage kidney disease. The allograft allocation system considers immunological factors and time in waiting list, but not the age of donors or recipients, in spite that both have prognostic relevance. Aim: To study the fairness of the allograft allocation system considering the age of donors and recipients. Material and methods: Based in the age distribution of donors and waiting list of patients for kidney transplantation, we constructed probability functions for receiving a graft as an allocation system based on age changes from a strict to a more lax criteria. Finally, we compared our model with the real transplants performed during 2012. Results: The probability distribution for receiving a kidney transplantation is displaced to the right compared with the age distribution of graft donors. This gap increases as the procuring hospitals enlist progressively older graft candidates without accepting older donors in parallel. An allocation system that maintains age parity between donors and recipients is fairer that other that allows lax criteria for the same allocation. This phenomenon is attenuated if the procurement acceptance criterion allows older donors. Conclusions: It is necessary to reduce the gap between donor and recipient ages for kidney transplantation to improve the fairness of the graft allocation system.