Final answer:
The thymus is the immune organ that reduces in size with age and involves the production and maturation of lymphocytes, particularly T cells, and secretes thymosins for immune response development.
Step-by-step explanation:
The organ of the immune system that shrinks in size as people age and is responsible for the production and maturation of lymphocytes is the thymus. During infancy and childhood, the thymus is much larger and more active but begins to atrophy as we age. The thymus is a bi-lobed organ that lies behind the sternum and is crucial for the development of T lymphocytes, or T cells, which are essential for the adaptive immune system. Lymphocytes are produced in the bone marrow and then mature either there (in the case of B cells) or in the thymus (for T cells). The thymus creates a group of hormones called thymosins that contribute to the development and differentiation of the immune response.