T-cells mature in the thymus gland, but the gland itself shrinks (involution) with age, starting after puberty, which reduces its output of new T-cells and leads to a decline in immune function, though the thymus produces all needed T-cells by puberty, and the aged immune system relies on peripheral T-cell maintenance and some extrathymic sites. This age-related atrophy, where fat replaces thymic tissue, affects the body’s ability to fight new infections and increases susceptibility to diseases