Why does thymus gets smaller as we age?

The immune system undergoes dramatic changes with age - the thymus involutes, particularly from puberty, with the gradual loss of newly produced naive T cells resulting in a restricted T cell receptor repertoire, skewed towards memory cells. Coupled with a similar, though less dramatic age-linked decline in bone marrow function, this translates to a reduction in immune responsiveness. The thymic involution, the shrinking of the thymus with age, resulting in changes in the architecture of the thymus and a decrease in tissue mass.

Though the thymus is fully developed before birth newborns have an essentially empty peripheral immune compartment immediately after birth. Hence, T lymphocytes are not present in the peripheral lymphoid tissues, where naïve, mature lymphocytes are stimulated to respond to pathogens. In order to populate the peripheral system, the thymus increases in size and upregulates its function during the early neonatal period.

Moreover, most immunity is produced during infancy and childhood, where it is very much needed because the immune system is still weak. Thus, it gradually decreases in size as we age because it is no longer needed that much.