There was more time between the Stegosaurus and the Tyrannosaurus Rex than between the Tyrannosaurus Rex and you. The Stegosaurus lived about 150 million years ago, while the T-Rex lived only about 67 million years ago – about 83 million years separated the two! The so-called Age of Mammals, which began when the non-avian dinosaurs were wiped out, has been going on for about 66 million years.