I don't think anything happens in the world outside your fortress, not yet. So it must have been something in your fort that brought about the new age. What probably happened is that goblins already were dominant, but as long as there were some powerful creatures (titans/megabeasts), the game mechanics didn't allow the goblin age to come. What was the name of the previous age?
The prev. age was called Age of Heroes. But I reckon it is all clear to me now. The world was generated with size set to pocket, number of civs,beasts,savagery to very high and minerals to very rare. Humans were already extinct by age of 250, the only civs left were kobs, dwarves, elves and goblins.
For added challenge, I chose to embark in freezing tundra, aquifier, saltwater, untamed wilds. Obviously, such setup was pretty inhospitable to large scale-living, something migrants couldn't grasp while coming in mobs and bringing their derpy grazing pets with them. Brief 7 year history of Sarveshedud, the aboveground fort:
~Embark. The only miner encases himself in ice while digging 5x5 pond. Surface has no trees.
~Disassembled the wagon, built depot. Got the business moving. Fort grows to 136 very fast.
~Vucar Shemamkol, the pet cow calf starves to death
~128 dwarves killed in a tantrum spiral. Number is precise, because I had to personally crush all of them under cave-in. 8 survived, though.
~Stone is found. Hurray!
~up to 10 miners encased in ice while digging the stone, and subsequently breaching the aquifier.
~90+ dead in goblin siege led by clown (bull fiend, bloated, beware the sting!)
~up to 30 dwarves dead in an ambush trying to collect last year goblinite.
~Queen arrives
~5 dwarves executed for not producing aluminium statues.
~Current fort population: 112
With death toll of 250+ (counting various misc. deaths), and migrants STILL coming in clusters of 2 to 5, it's no wonder goblins dominate. I guess I am to blame...