Short answers, pulled from the story.
The first record of body armor in history was found on the Stele of Vultures in ancient Sumer, located in what is now south Iraq. This early evidence of protection predates written history and marks the dawn of a technological race that would span millennia.
Mail, sometimes called chainmail, is believed to have first appeared some time after 300 BC and its invention is credited to the Celts. The Romans later adopted the Celtic design, integrating the interlocking iron rings into their own military machine.
Plate armor became cheaper than mail by the 15th century, a shift driven by the economic devastation of the Black Death which made labor much more expensive. This demographic catastrophe forced armorers to rethink their methods, leading to the development of water-powered trip hammers that made the forming of plates faster and cheaper.
Full suits of armor were actually worn by generals and princely commanders right up to the second decade of the 18th century. This was the only way they could be mounted and survey the overall battlefield with safety from distant musket fire.
In Japan, armor continued to be used until the late 19th century, with the last major fighting in which armor was used occurring in 1868. Samurai armor had one last short lived use in 1877 during the Satsuma Rebellion, a final stand of a warrior class that had once dominated the landscape of feudal Japan.
The first ironclad battleship, with iron armor over a wooden hull, was launched by the French Navy in 1859. The following year they launched, which was twice the size and had iron armor over an iron hull, marking the beginning of a new era in naval warfare.