*tosses headcanon around*

Filly- Celestia, known as Sun Petal when she was younger, was just a normal unicorn.
Teen Mare- Normal teenaged mare. Around Twilight's age at this time.
Teen Mare; Rising to Power- Sun Petal and her sister Moondust discover the Tree of Harmony and learn how to harness its powers. Sun also finds out she is also a very powerful magic-user.
Mare; Alicorn Transformation- Sun and her sister became alicorn goddesses (and soon after, the Princesses of Equestria) once they discovered their true purpose of guarding the land.
Mare; Pre-Godhood- Now being known as Celestia, this new princess is still learning the ways of Equestria.
Mare; Godhood- Now a fully grown goddess, Celestia and Luna rule in harmony for thousands of years. This is the stage when Luna becomes corrupt (Nightmare Moon) and she is banished to the moon.