By age 75, the thymus is little more than fatty tissue. Fortunately, the thymus produces all of your T cells by the time you reach puberty. They are long-lived and that's why you can lose your thymus without impairment of your immune system.

Thymic output of T cell repertoire during the growing phases is vital, but it becomes unnecessary for repertoire maintenance during adulthood. This happens because the T cell regeneration in adulthood is almost entirely derived from homeostatic proliferation of the EXISTING T cell pool, which is sufficient to maintain a large compartment of naive CD4 T cells. Thymic T cell generation can add new naive T cells and enrich diversity, while homeostatic T cell proliferation can sustain the richness of the TCR repertoire already created. This means that the Thymic lymphocytes produced before thymectomy are long-lived naive T cells, which are maintained stable in quantity thanks to Homeostatic proliferation in adulthood.