The First Age began when the Masters returned to their own private realms, content they had given the citizens of the Inner Realm enough support they would be able to create their own society.
Dedede stepped up to become king, and decided to begin by constructing a magnificent castle that he hoped would raise the morale of everybody, who was disappointed that the Masters had left them. When the construction was completed, the effect was as intended. He named the castle Devotee Castle, as he considered it a sign of devotion to the Masters that the people they had created a world for would be able to live in peace. Soon after, Devotee City was built around the castle.
The age's end began when Dedede's advisor, Bandanna Dee, mysteriously disappeared on the twenty-second anniversary of the construction of Devotee Castle. Dedede became paranoid, and without Bandanna Dee's economic assistance his kingdom began to collapse. The kingdom's wealth steadily declined over a period of ten years until the majority of citizens found themselves in poverty, leading to Micool the First starting a rebellion which successfully overthrew Dedede and began the Second Age.
The Second Age began when Micool the First stepped up to become the new king. He managed to recover the state of the kingdom (although it didn't reach the same state it attained at the peak of Dedede's reign), and began to focus on expanding the military, knowing that during the depression many settlements had declared themselves independent to Devotee, and planning to reclaim the land that had been lost.
Despite his best efforts, Micool's plans never reached fruition during his lifetime. After dying of old age, his son Micool the Second became the new king, and he successfully completed his father's plan, even taking over certain nations that had never been apart of the Devotee kingdom.