Senescence or biological aging impacts a vast variety of molecular and cellular processes. To date, it is unknown whether CD4+ Th cells display an age-dependent bias for development into specific subpopulations. In this study, we show the appearance of a distinct CD4+ T cell subset expressing IL-4 at an early stage of development in infant adenoids and cord blood that is lost during aging. We identified by flow cytometric, fluorescent microscopic, immunoblot, and mass spectrometric analysis a population of CD4+ T cells that expressed an unglycosylated isoform of IL-4. This T cell subpopulation was found in neonatal but not in adult CD4+ T cells. Furthermore, we show that the mRNA of the Th2 master transcription factor GATA3 is preferentially expressed in neonatal CD4+ T cells. The Th2 phenotype of the IL-4+ CD4+ T cells could be reinforced in the presence of TGF-β. Although the IL-4+ CD4+ T cells most likely originate from CD31+ CD4+ T recent thymic emigrants, CD31 was downregulated prior to secretion of IL-4. Notably, the secretion of IL-4 requires a so far unidentified trigger in neonatal T cells. This emphasizes that cytokine expression and secretion are differentially regulated processes. Our data support the hypothesis of an endogenously poised cytokine profile in neonates and suggest a link between cytokine production and the developmental stage of an organism. The determination of the IL-4 isoform–expressing cells in humans might allow the identification of Th2 precursor cells, which could provide novel intervention strategies directed against Th2-driven immunopathologies such as allergies.