 The smallest known Tyrannosaurus rex individual ( LACM 28471 , the " Jordan theropod " ) is estimated to have weighed only 29 @.@ 9 kilograms ( 66 lb ) at only 2 years old , while the largest , such as FMNH PR2081 ( " Sue " ) most likely weighed about 5 @,@ 654 kg ( 12 @,@ 465 lb ) , estimated to have been 28 years old , an age which may have been close to the maximum for the species . T. rex juveniles remained under 1 @,@ 800 kg ( 4 @,@ 000 lb ) until approximately 14 years of age , when body size began to increase dramatically . During this rapid growth phase , a young T. rex would gain an average of 600 kg ( 1 @,@ 300 lb ) a year for the next four years . This slowed after 16 years , and at 18 years of age , the curve plateaus again , indicating that growth slowed dramatically . For example , only 600 kg ( 1 @,@ 300 lb ) separated the 28 @-@ year @-@ old " Sue " from a 22 @-@ year @-@ old Canadian specimen ( RTMP 81 @.@ 12 @.@ 1 ) . This sudden change in growth rate may indicate physical maturity , a hypothesis which is supported by the discovery of medullary tissue in the femur of an 18 @-@ year @-@ old T. rex from Montana ( MOR 1125 , also known as " B @-@ rex " ) . Medullary tissue is found only in female birds during ovulation , indicating that " B @-@ rex " was of reproductive age . 
