The smallest known "Tyrannosaurus rex" individual (LACM 28471, the "Jordan theropod") is estimated to have weighed only at only 2 years old, while the largest, such as FMNH PR2081 ("Sue"), most likely weighed about , 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 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 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 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 that 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.