The Iron Age was the third and last period in the prehistory of humankind on Earth, following the Bronze Age. (PROSE: A History of Humankind [+]Official Guides (BBC Children's Books, 2016).)
Whilst in the 2nd century, the Twelfth Doctor identified cairns as Iron Age churches constructed by the Picts. (TV: The Eaters of Light [+]Rona Munro, Doctor Who series 10 (BBC One, 2017).)