Quote from the opening of Friendship is Magic, which sets up the history of Equestria for the rest of the storyline:
Once upon a time, in the magical land of Equestria, there were two regal sisters who ruled together, and created harmony for all the land. To do this, the eldest used her unicorn powers to raise the sun at dawn; the younger brought out the moon to begin the night. Thus, the two sisters maintained balance for their kingdom and their subjects, all the different types of ponies. But as time went on, the younger sister became resentful. The ponies relished and played in the day her elder sister brought forth, but shunned and slept through her beautiful night. One fateful day, the younger unicorn refused to lower the moon to make way for the dawn. The elder sister tried to reason with her, but the bitterness in the young one’s heart had transformed her into a wicked mare of darkness: Nightmare Moon.
She vowed that she would shroud the land in eternal night. Reluctantly, the elder sister harnessed the most powerful magic known to ponydom: the Elements of Harmony. Using the magic of the Elements of Harmony, she defeated her younger sister, and banished her permanently in the moon. The elder sister took on responsibility for both sun and moon and harmony has been maintained in Equestria for generations since.
Later it’s established that Princess Celestia, who seems for all intents and purposes the ruler of Equestria, is the sister who raised the sun, and Princess Luna who raised the moon and was banished for a thousand years. Therefore Celestia and Luna must both be at least a thousand years old.
The fanon response to this thus far has been to think of princesses as immortal. This seems reasonable, but there are some slight (not insurmountable) problems with this conception. The main inconsistency is that in A Canterlot Wedding, there’s a flashback sequence in which we see Princess Cadance foal-sitting Twilight Sparkle, in which she's visibly younger than in later appearances of her in the ‘present day’:
Presumably the former shows her at present-day Twilight’s age — perhaps a young adult, 18–22 in human years — or maybe a little younger, while the latter shows her as a full-grown mare alicorn.
It’s unlikely mathematically that Twilight just happened to be born 10 (human) years or so after Cadance if princesses are immortal: forever is a long time, after all. So we must assume that Cadance, at least, is mortal, or else Twilight was extremely lucky to have been born at a time when her foal-sitter was a teenage princess. (Naturally, we don’t know how many princesses there are, but if there were that many we’d have been sure to have met more than three by now.)
Perhaps it’s only Celestia and Luna who are immortal? (We can presume that immortal ponies reach a certain stage of physical growth then stop, otherwise Celestia would look like Granny Smith times a hundred.) They must have lived at least a thousand years, after all, since that’s how long Luna was imprisoned in the moon. However, The Crystal Empire provides some interesting challenges. The Empire was also gone a thousand years. It would seem that it disappeared shortly after Luna was banished, since it re-appeared shortly after she returned and they were both apparently gone for the same amount of time.
One could assume that Celestia and Luna were responsible for defending the Crystal Empire. However, if they were, they could’ve used the Elements of Harmony to defeat King Sombra. This suggests that the Crystal Empire is not part of Equestria, but there must be an important magical link between the Empire and Equestria: “If the empire is filled with hope and love, those things are reflected across all of Equestria. If hatred and fear take hold …”
There’s another problem with the explanation that Celestia and Luna were defending the Crystal Empire: Celestia claims that “even [her] knowledge of the Empire is limited.” Why would you not know anything about a place you’re supposed to be defending? For that matter, you might ask why the princess of a land would not know anything of an Empire with which hers is magically linked. But perhaps the link was not known of until King Sombra came to power and hatred and fear took hold, and its effects were noticed in Equestria.
Further evidence that the Crystal Empire is not part of Equestria comes when the crystal ponies call Cadance “the crystal princess” when she rescues the Crystal Heart from falling into the hands of King Sombra and restores it to its rightful place. Since Cadance is only 10 human years or so older than Twilight, she can’t have held that title back when the Empire was first flourishing. Perhaps she was created the crystal princess in recognition of her protection of the empire and her saving of the heart.
Are the crystal ponies immortal? It’s possible. One of them asks: “Have we really been gone a thousand years? It feels like it was just yesterday.” More likely, though, is that King Sombra’s curse caused time to freeze or be slowed significantly within the Empire, and the ponies are simply experiencing this, much like the lack of sense of time we have when we sleep, explaining why it ‘feels like it was yesterday.’
Equestrian history starts with three tribes: unicorns, pegasi, and Earth ponies. The Earth ponies grew food and gave some to the pegasi and unicorns, in exchange for weather and night & day, respectively. The tribes distrusted one another. One day a snowstorm arrived, preventing the Earth ponies from growing food. They were unable to come to an agreement about what to do about it. Eventually the ponies found that the storm was caused by windigos. In light of this, the ponies stayed together, declaring new friendship with each other. When the ice had melted, the tribes worked to form a new land, Equestria.
At some point, Discord rose to power, ruling with chaos and disharmony, changing the night and day at random, and causing things on Earth to happen without reason. Celestia and Luna, defeated him using the Elements of Harmony, the world’s most powerful magic, using them to turn him into stone. Thereon they ruled in peace: Celestia solely responsible for day, and Luna night, until Luna tried to make it night forever, but Celestia again used the Elements to defeat her sister and banish her to the moon.
Shortly thereafter, the Crystal Empire was overtaken by King Sombra, who enslaved its citizens. He was defeated, but he placed a curse on the Crystal Empire which caused it to disappear from the land.
For about a thousand years, nothing interesting happened.
At the thousandth summer sun celebration after Luna was banished, the spell which trapped her in the moon was broken, and she returned, again with the intention of making the night eternal. She attempted to destroy the Elements, but they were remade when the ‘mane six’ re-conjured them. They defeated Luna and she swore off her past evils and once again took up the role of raising the moon as normal.
A while later, Discord returned (due to Celestia and Luna’s spell on him being broken, because they no longer controlled the Elements: the mane six did) but was defeated again by the Elements of Harmony, this time wielded by the mane six.
Shortly thereafter, a thousand years after it vanished, the Crystal Empire re-appeared, along with King Sombra. Princess Cadance, of Equestria, was able to protect the Empire from him for a time, but she grew weaker, it became necessary for the Empire to defend itself. Once the Crystal Heart was found and powered, Sombra was defeated and the magical link with Equestria was renewed.