Phenotypic maturation of RTEs occurs with delayed kinetics in aged compared with young mice. Splenocytes and thymocytes from mice of the indicated ages were stained for CD4, CD8, CD24, and Qa2 surface expression. GFP+ CD4+ and CD8+ SP thymocytes (black histograms) and splenocytes gated as GFPhi (green histograms), GFPlow (pink histograms), and GFPneg (blue histograms) were analyzed for CD24 and Qa2 expression. Data are representative of seven or more mice analyzed from each age group in five independent experiments.