Mesozoic Era includes the Triassic, Jurassic, and Cretaceous Periods. It was concluded by an enormous meteorite impact that caused a mass extinction that wipe out the dinosaurs and up to 80% of life on Earth. This period spans from about 252 million years ago to about 66 million years ago which is also recognized as the age of reptiles or the age of dinosaurs.
Answer - c. 66 million years ago.