<P> The thymus is largest and most active during the neonatal and pre-adolescent periods . By the early teens, the thymus begins to atrophy and thymic stroma is mostly replaced by adipose (fat) tissue . Nevertheless, residual T lymphopoiesis continues throughout adult life . </P> <P> In children, the thymus is pinkish - gray, soft, and lobulated on its surfaces . At birth it is about 4--6 cm long, 2.5--5 cm wide, and about 1 cm thick . It is made up of two lobes that meet in upper midline, that stretch from below the thyroid in the neck to as low as the cartilage of the fourth rib . It lies beneath the sternum, rests on the pericardium, and is separated from the aortic arch and great vessels by a layer of fascia . The left brachiocephalic vein may even be embedded within the thymus . In the neck, it lies on the front and sides of the trachea, behind the sternohyoidei and sternothyreoidei . </P> <P> The thymus enlarges during childhood, and atrophies at puberty . Unlike many other organs, the thymus is at its largest in childhood . The thymus reaches maximum weight (20 to 37 grams) by the time of puberty . The thymus of older people is scarcely distinguishable from surrounding fatty tissue . With aging the thymus slowly shrinks, eventually degenerating into tiny islands of fatty tissue . By the age of 75 years, the thymus weighs only 6 grams . In children the thymus is grayish - pink in colour and in adults it is yellow . </P> <P> The thymus consists of two lobes, merged in the middle, surrounded by a capsule that extends with blood vessels into the interior . The lobes consists of a dense outer cortex and an inner less dense medulla . The lobes are divided into smaller lobules 0.5 - 2mm diameter, between which extrude radiating insertions from the capsule along (septa). </P>

At what age does the thymus reach its maximal size