Golden Age, Silver Age, Bronze Age, Heroic Age, Iron Age. Everything going progressively downhill, people being less noble and weather worse, no good music anymore, hey you kids get off my lawn. It's a basic human view of the past that turns up all over. My guess is it's a psychological expression of childhood being wonderful and secure compared to having to get out and work for a living.
Greek mythology (well, almost any mythology) is such a cobbled-together mess of various stories from different eras and regions, contradicting each other and not even consistent themselves despite our efforts to figure it out. Hey! It's like comics....