Having access to metal and proper weapons doesn't apply to worldgen, only in Fort and Adventurer mode kobold encounters. For worldgen, there are a number of approaches.
First, make kobolds huge for worldgen, then make 'em normal sized afterwards. Creature size means a lot in worldgen whenever someone gets in a fight. Random deaths can still occur though.
Second, kobolds don't really expand from their cave homes. To let 'em spread around, change their home type from cave to anything else, really. Forest homes or cities would be the fastest, the default biome likes of kobolds allows for fast spread of their civilization.
You could limit the other civs to some specific rare biomes, they'd be stuck in relatively small areas then while kobolds would have access to more land to conquer. To get the Age of Kobold though, it'd require that a lot of the titans, semimega- and megabeasts would be dead though(I think, pretty sure on it anyway).
All that AoK needs is for kobolds to be the most numerous civ anyway. Normally this would be pretty rare, obviously.
Age of Heroes: I recall this is what happens when there are a lot of good sized civs around and a few amount of megabeasts left, like two or three. Demon overlords may count for this.
Age of Civilization: Semimega- and megabeasts are dead, titans too. This happens when there are a lot of civilizations around and none is overwhelmingly populated compared to the rest.
Age of Death: Everything is DEAD. And I mean everything, everything bigger from insects. Since bugs are nearly limitless in number, mod them out along other vermin, then have creature lifespans ridiculously low. 1-2 years?