Age of Steel

The Age of Steel was the name given by John Lumic, as Cyber-Controller, to the time when the Cybermen would upgrade and replace the humans of Pete's World as Human.2. (TV: The Age of Steel)

Information from invalid sources
In an altered state of reality caused by the First's Reality Virus that had been upgraded by the CyberReaper, when the Cybermen took over Earth, they declared that they were now in the "Age of Steel". (GAME: The Edge of Reality)