why the size of thymus gland decreases as the age of a person increases ?

The main function of the thymus gland is to produce T cells during childhood so that it can attack the foreign cells. Once the person reaches puberty the thymus starts shrinking as the main role is completed during childhood years and it is not longer needed for the further growth. It is also said that that the sex hormones which are secreted at the time of puberty also trigger the gland to shrink. 

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