Founding studies of cellular immunology emphasized that tolerance to allografts could only be achieved early in the embryonic or neonatal period, suggesting that the establishment of self-tolerance, a main event in the organization of the immune system, would necessarily take place in immature hosts. Contradicting these ideas, oral tolerance is a common, daily phenomenon, easily achieved by a physiological route in adult immunocompetent animals. Furthermore, there is solid evidence that, after the neonatal period, the susceptibility to oral tolerance induction also wanes and that it may be restored by adoptive transfer of cells from young hosts. These findings are briefly reviewed here to emphasize that immunological activity is a continuous and ongoing epigenesis extending throughout the entire life of the organism, far beyond the early phases of ontogenesis.