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Analysing T‐cell receptor excision circle numbers in healthy individuals we find a marked change in the source of naive T cells before and after 20 years of age. The bulk of the naive T cell pool is sustained primarily from thymic output for individuals younger than 20 years of age whereas proliferation within the naive phenotype is dominant for older individuals. Over 90% of phenotypically naive T cells in middle age are not of direct thymic origin. Moreover, this change in source of naive T cells is accompanied either by an increased death rate of T cells from the thymus or reduced thymic export. Modelling of these processes shows that new naive T cells of a thymic origin have a half‐life of approximately 50 days before this change occurs, and that either the life‐span of recent thymic emigrants (but not necessarily of all naive cells) decreases approximately threefold in middle age, or thymic production drops by this …