Final answer:
The decline in immune system function with age is known as immunosenescence, which becomes noticeable around age 50 and is related to the process of thymic involution that affects T cell maturation.
Step-by-step explanation:
The decline in the functioning of the immune system during aging is referred to as immunosenescence. This phenomenon generally begins at about age 50 and becomes more pronounced with increasing age. As a result, older individuals are often more susceptible to diseases. Immunosenescence is a natural part of the aging process and is a significant factor in the increased occurrence of infections and chronic diseases among the elderly population. One main cause of immunosenescence is the process of thymic involution, which is the gradual shrinking of the thymus gland starting from birth. The thymus gland is essential for the maturation of T cells, which are vital components of the immune system. After the age of 35-45, thymic involution continues at a slower rate until very old age, contributing to the decline in immune system efficiency.