At what point in life is the mass of lymphoid tissue said to decrease?

The mass of lymphoid tissue is known to decrease after adolescence. This is because, during adolescence, the immune system matures and the production of certain lymphoid tissues, such as the thymus, begins to decline. The thymus is particularly important for T cell development, and as individuals transition into adulthood, it gradually becomes less active and atrophies, leading to a reduction in overall lymphatic tissue mass.

In contrast, lymphoid tissue is generally more abundant during infancy and childhood as the immune system is actively developing. Throughout adulthood, while there may be some involution of certain lymphoid components, the most significant decrease occurs post-adolescence, making this stage a key period in the understanding of immune system evolution. Old age does see some changes in the immune system, but the marked decrease is typically recognized to start after adolescence.