Why does the thymus gland shrink?

According to InnerBody, the thymus shrinks from puberty onwards as its main role is completed during the childhood years. The main function of the thymus gland is to train immature T cells that are produced in the red bone marrow to become mature T cells that attack only foreign cells.

The thymus grows throughout childhood until the onset of puberty. InnerBody states that that the immune system creates most of its T cells during childhood. The immune system requires very few new T cells after puberty. Because the body no longer has the same need for new T cells, the thymus gland is no longer needed to the same extent.