Which gland decreases in size with age?

The thymus gland is known to decrease in size with age, a process that is an essential aspect of human development and immune function. A significant portion of the thymus gland is active during childhood and adolescence, as it plays a critical role in the maturation of T lymphocytes, which are vital for the adaptive immune system. As an individual ages, the thymus undergoes involution, gradually shrinking and being replaced by fatty tissue, which leads to a decrease in the production of thymic hormones that are important for immune response.

This decline in thymic function can contribute to the elderly being more susceptible to infections and having a reduced immune response compared to younger individuals. In contrast, other glands listed, such as the adrenal gland, pituitary gland, and thyroid gland, do not show the same reduction in size with aging; in fact, some may maintain or even increase in activity or size in certain contexts. Thus, the uniqueness of the thymus gland's shrinkage with age distinctly identifies it as the correct answer.