The thymus reaches its maximum size early in life and then begins to shrink, producing fewer T cells with increasing age. This thymic decline is thought to contribute to age-related T cell lymphopenias and hinder T cell recovery after bone marrow transplantation. Although several cellular and molecular processes have been implicated in age-related thymic involution, their relative contributions are not known. Using heterochronic parabiosis, we observe that young circulating factors are not sufficient to drive regeneration of the aged thymus. In contrast, we find that resupplying young, engraftable thymic epithelial cells (TECs) to a middle-aged or defective thymus leads to thymic growth and increased T cell production. Intrathymic transplantation and in vitro colony-forming assays reveal that the engraftment and proliferative capacities of TECs diminish early in life, whereas the receptivity of the thymus to TEC engraftment remains relatively constant with age. These results support a model in which thymic growth and subsequent involution are driven by cell-intrinsic changes in the proliferative capacity of TECs, and further show that young TECs can engraft and directly drive the growth of involuted thymuses. Copyright © 2015 by The American Association of Immunologists, Inc.