The thymus gland (which generates T cells) is a rare organ in that it that is biggest in children and starts to atrophy after puberty becoming little more than fatty tissue in old age. It used to be assumed that thymic output (new T-cells) was negligible after adolescence and this was part of the reason the adaptive immune system declines with age. However as usual the truth is more complex. Studies like this one described below suggest that thymic output is at least in part environmentally influenced. In this study, comparing older cyclists with an age-matched sedentary group and a young cohort, the authors found the older cyclists had almost as many new T-cells as the young cohort and significantly more than the older sedentary group.
https://www.ncbi.nlm.nih.gov/pubmed/29517845
