[quote=@SleepingSilence]
[@Frizan] Is the metal age defined in any way? Like is this based upon anything? Or can somebody construct this world, its technology, setting and the behavior of these "red-eyed" constructs in basically anyway they wanted?
[/quote]

In this instance the "metal age" is defined as an invading force. It could be a literal army, or it could even be a metaphorical "invasion", showing the change from a "natural" existence to an automated/industrial one.