Tyrannosaurus Rex Was Probably a Late Bloomer—and May Have Taken Around 40 Years to Grow Up

The behemoth dinosaurs grew more slowly and had longer life spans than previously thought, a new study suggests

Sarah Kuta | Daily Correspondent

January 26, 2026

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Over the years, Tyrannosaurus rex has solidified its reputation as one of the most formidable predators to ever roam the Earth. But according to new research, these ferocious beasts probably weren’t full-grown until around 40 years old, meaning they spent much of their lives as smaller, less intimidating versions of the giants we imagine.

With a new paper published January 14 in the journal PeerJ, scientists provide an updated timeline for the extinct species’ growth and development. Their investigation suggests that T. rex—a name that means “king of the tyrant lizards”—probably matured more slowly than previously thought.

“It took the prince a lot longer to grow into the king,” says Steve Brusatte, a paleontologist at the University of Edinburgh in Scotland who was not involved with the research, to Scientific American’s Cody Cottier.

Previous research, conducted in the early 2000s with a few fossils, suggested that the legendary dinosaurs reached their full size—up to around 20,000 pounds and 40 feet long—between 20 and 25 years old, and could live up to about age 30. Since then, however, additional T. rex bones have surfaced, which means scientists now have access to a larger sample size.

For the study, scientists investigated the tissue microstructures of the thigh and shin bones from 17 individuals of various ages. More specifically, they looked at the growth rings that formed annually in the T. rex bones—similar to the rings of a tree.

“Leg bones tend to be one of the fastest growing bones in an animal, because they have to compensate for the weight as the animal is getting bigger,” says study co-author Holly Woodward, a paleohistologist at Oklahoma State University, to Science’s Jake Buehler. “So, it’s going to tell us the most about how the animal was growing from year to year.”

They cut ultrathin slices from the bones, then counted all the growth rings they could see—including groups of narrow rings bunched together that were overlooked in previous studies. They found even more rings when they looked at the samples using a special polarized-light technique.

Then, they combined the data to produce a new, detailed growth trajectory for the species. “The composite growth curve provides a much more realistic view of how Tyrannosaurus grew and how much they varied in size,” study co-author Nathan Myhrvold, a mathematician and the founder and CEO of Intellectual Ventures, tells IFLScience’s Stephen Luntz.

Growth rings appear to have stopped forming in T. rex leg bones after about 40 years, which suggests this is the age the reptiles reached maturity—if they managed to live that long. Most probably died before they ever reached their full adult size, according to the researchers.

Two of the specimens in the study grew more slowly than the others, which hints that they may have belonged to a different species—possibly the controversial and diminutive Nanotyrannus lancensis described late last year. It’s also possible, however, that those individuals were simply sick or injured, or that some environmental hurdle stunted their growth.

Overall, the results offer fresh insights into what life may have been like for the legendary species, which lived during the Late Cretaceous period, some 66 million to 68 million years ago. For much of its life, the findings suggest, a T. rex roamed as a small or medium-sized creature, and probably had to compete for food. Only once it reached its full, gargantuan proportions did it become an apex predator, the findings indicate.

The results may also prompt scientists to revisit the life cycles of other extinct species.

“I expect that the estimates of growth curves of other dinosaurs will now have to be revisited,” says Thomas Carr, a paleontologist at Carthage College who was not involved with the research, to New Scientist’s James Woodford. “Overall, we’ll see a shift in our understanding of dinosaur development across the board.”

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.