From John Noble Wilford in The New York Times:
Paleontologists now think they have the answer: a teenage growth surge in which a T. rex normally put on an average of 4.6 pounds a day over four years.
A new study of tyrannosaur bones, scientists reported yesterday, has determined that from about 14 to 18 years of age, a meat-eating T. rex with a humongous appetite gained about 6,600 pounds to reach its full adult weight of more than 11,000 pounds, length of 42 feet and height of about 14 feet at the hips.
These dinosaurs were at least 15 times the size of the polar bear, today’s largest living terrestrial carnivore, and were about the same weight as an African bull elephant.