How Old Were the Oldest Dinosaurs?

dinosaurs, lifespan
From research, paleontologists have determined that the T. Rex grew very fast and reached adulthood around age 20. (Image credit: Nobumichi Tamura / Natural History Museum.)

Dinosaurs are keeping mum on exactly how many birthdays they celebrated back in the day.

But scientists have come up with clever ways to guess the lifespans of the ancient reptiles.

To determine how long and how fast Tyrannosaurus rex grew, paleontologist Mark Norell at the American Museum of Natural History and colleagues counted rings in some of the dinosaurs' ancient bones. Similar to tree rings, each ring in a dinosaur bone marks another year of growth.

“We found on the basis of seven specimens that T. rex reached adult size at 19 years of age,” said Norell. “The oldest T. rex that we could find was the specimen named Sue at the Field Museum in Chicago, which was 28 years old. T. rex grew very fast — for a four-year stint, it was putting on more than 5 pounds per day.”

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Corey Binns lives in Northern California and writes about science, health, parenting, and social change. In addition to writing for Live Science, she's contributed to publications including Popular Science,, Scholastic, and the Stanford Social Innovation Review as well as others. She's also produced stories for NPR’s Science Friday and Sundance Channel. She studied biology at Brown University and earned a Master's degree in science journalism from NYU. The Association of Health Care Journalists named her a Centers for Disease Control and Prevention Health Journalism Fellow in 2009. She has chased tornadoes and lived to tell the tale.