It is unclear how the immune response in early life becomes appropriately stimulated to provide protection while also avoiding excessive activation as a result of diverse new antigens. T cells are integral to adaptive immunity; mouse studies indicate that tissue localization of T cell subsets is important for both protective immunity^,,, and immunoregulation^,. In humans, however, the early development and function of T cells in tissues remain unexplored. We present here an analysis of lymphoid and mucosal tissue T cells derived from pediatric organ donors in the first two years of life, as compared to adult organ donors, revealing early compartmentalization of T cell differentiation and regulation. Whereas adult tissues contain a predominance of memory T cells^,, in pediatric blood and tissues the main subset consists of naive recent thymic emigrants, with effector memory T cells (T_EM) found only in the lungs and small intestine. Additionally, regulatory T (T_reg) cells comprise a high proportion (30–40%) of CD4⁺ T cells in pediatric tissues but are present at much lower frequencies (1–10%) in adult tissues. Pediatric tissue T_reg cells suppress endogenous T cell activation, and early T cell functionality is confined to the mucosal sites that have the lowest T_reg:T_EM cell ratios, which suggests control in situ of immune responses in early life.