Scientists do not know the exact lifespan of dinosaurs, but they estimate that they lived about 75 to 300 years. Animal lifespans relate in part to their body size and in part to their type of metabolism. Their possible maximum age can be estimated from the maximum lifespans of modern reptiles, such as the 66-year lifespan of the common alligator and the impressive lifespan of some tortoises.

One specimen of the now-extinct Black Seychelles Tortoise, which was an adult when captured, lived a record 152 years in captivity (1766-1918) and had an accidental death. These estimates, based on lifespans of cold-blooded animals, would be too long if dinosaurs had metabolisms more similar to modern birds and mammals.