The Age of the Dinosaurs

I hope this doesn't sound too patronising, but the use of the capital letter for Age suggests that the question setter thinks he or she is using a scientific term. I don't think this is the case. The Mesozoic is (or was) an era; the Triassic, Jurassic and Cretaceous are (or were) periods, as the setter explains in the answer.

I don't have a problem with anyone using the informal term 'age' to refer to either an epoch, a period, an era or an eon. But if you want to be precise, the Mesozoic is an era.