In my world, there are three distinct ages, each one defined by a major event.

First Age: Sunset The world existed as it did today. With relatively advanced technology, an interconnected world and all the other problems and benefits of the world today. It made prime conditions for the spread of Kumamoto disease, which culminated on July 3rd into the Sunset over Kumamoto.

Second Age: Moon Following nuclear annihilation, the world struggled to rebuild. Since my story focuses on Fallen Japan, the country's government fell apart. Kyushu itself received the forefrunt of the assault and begins to develop a seperate identify to the more intact mainland.

Clans formed. You have far too many to name, but the two I'm focused on are the Akinokodomo (Autumn's Children's - think Little Lamplight, FO3) and the Claydolls. The Claydolls started out as the apocalypse's solution to the postal service: need thing getting from A to B? Then the Claydolls can do it. The Claydolls are tough motherfuckers - don't fuck with the Claydolls, because those stories you hear about torture are all completely true. One Claydoll, Kiyotala Tabitate, is captured and tortured by the Akinokodomo, which sparks a war between the two factions and eventually the Claydolls win. Their power consolodates into the vacuum left after the government fell apart. The Japanese government had a better time reforming than it really should have after the war, and dislikes the Claydolls and Kyushun independance.

This all takes place over 5 or so years.

Hundreds of years later, the world is healed and the nations of the world are gone. Nihongo and Higo form the two japanese islands, with Songzhan forming Manchuria and Korea amd Sachaulou forming the Chinese east coast. Story happens and by the end of it, Songzhan is destroyed and so ends the Second Age.

Third Age: Sunrise Haven't set anything in the Third Age yet concretely but it's basically just post-modernism, like you would see in Mass Effect or Star Trek.