Age-related decline in immunity can impair cell-mediated responses during an infection, malignancy, and acute allograft rejection. Although much research has been allocated to understand the immune responses that impact the former two conditions, the cellular mechanisms by which aging impacts the immune acceptance of organ allografts are not completely clear. In this study, we examined how recipient age impacts the efficacy of therapies that modulate immune recognition of allografts using an immunogenic murine skin transplant model. We found that costimulatory blockade-based treatment failed to extend allograft survival in older recipients to the same extent as that observed in younger recipients. CD8+ T cells were critical for the inability of aged recipients to achieve maximal allograft survival. Although aged mice displayed a larger number of effector memory T cells prior to transplantation, these cells did not exhibit enhanced alloreactivity compared with young memory T cells. In contrast, naive aged CD8+ T cells exhibited enhanced IFN-γ production to allostimulation compared with young naive T cells. Our results provide evidence that aging enhances CD8+ T cell alloreactivity. This could impair the ability of costimulatory blockade-based therapies to prolong allograft survival. Thus, targeting CD8+ T cells in humans may be a way to improve outcomes in older patients requiring immune modulatory therapy.