He's asking how the game determines that the civs are collapsing, though.
Exactly. The game doesn't seem to track resource consumption or population constraints, and the only way to stop population growth is with killing, so with megabeasts removed from the picture, how can
all civs be in a state of collapse? If one was dominating the others in warfare, then it'd be the
Age of the Winning Side, right?
But something must be happening, because in long world-gens, we see the Twilight Age. Sometimes after a golden age, sometimes immediately after the last megabeast falls.
Theoretically, I guess, a period of mutual warfare could deplete everyone's population, and then the warfare ends (with no side really "winning") and everyone would look to be on the decline...but that would be a transient dark age, not a twilight age, because the population would always bounce back after a period without warfare, unless the world gen counts a lot of factors I don't think it does (yet).
EDIT:
So, there's an age of death. But what if someone made a lever operated pump in fortress mode that could flood the world, which was then operated in adventure mode, killing every land creature not underground? What would that be?
Strictly speaking, it would still be an Age of Death, unless the Dwarves held out (I don't have high hopes--the world gen'd Mountainhomes couldn't withstand a flood any better than a Dark Fortress, and a fortress mode fort would have to be abandoned and so wouldn't protect any dwarves), in which case it'd be the Age of Dwarves.
Truthfully speaking, it should be the Age Of The Merfolk's Ultimate Vengeance For All Those Air-Drowning Farms, You Sick, Sick, A-Holes.