Brewer's: Brazen Age
The age of war and violence. It followed the silver age.
To this next came in course the brazen age, A warlike offspring, prompt to bloody rage, Not impious yet. Hard steel succeeded then, And stubborn as the metal were the men.
Dryden: Metamorphoses, i.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894