It is the second historically defined period in the Land of Stories history. Many of the events that occurred during this period are known in our world as our most famous fairy tales. According to the Timeline Chart from A Grimm Warning, the Age of Magic lasts roughly from the Enlightenment to the end of the Industrial Age (17th-19th century), though time still moved a lot slower in the Land of Stories before Alex and Conner were born.
After the end of the Dragon Age, marked by the defeat of the dragons, the fairies formed the Fairy Council and founded the Happily Ever After Assembly. In A Grimm Warning, Empress Elvina tells the twins how the world was then divided up among the surviving human species, giving 'non-human' species (elves, trolls, goblins) considerably worse areas to live than humans and fairies. The kingdoms were formed.
During this time, many of the "fairy tale events" happen; the Eastern Kingdom falls into a hundred year sleep, Rapunzel is freed from her tower, etc. Somewhere during this age, the Snow Queen seizes hold of the Northern Kingdom and is eventually defeated and banished by Wise Prince White (Snow White's grandfather). The Enchantress tries to rise to power with the aid of Rumplestiltskin, but fails. Rumplestiltskin is imprisoned.
Prince Charlie Charming is turned into a frog and hides away in the Dwarf Forests; his brothers (Chance, Chase & Chandler) go looking for him and meet their brides (Cinderella, Sleeping Beauty &Snow White) during the search.
The Age of Magic is followed by the Golden Age.