whoawhoawhoa
clestria was a princess ever since she was born?
cool
no, that's just fanart.
There isn't anything known about Celestia and Luna's childhood. From what has been given throughout the show, they took control of Equestria at a certain point (not revealing spoilers in case you haven't seen the entire series yet). It is not known if they were in power prior to that point, or if a royal family was in place beforehand. However they were obviously in adulthood by this time.
it's possible they were born princesses, had their positions taken from them, and then re-took said positions. it's also possible they were simply made rulers of Equestria after a particular event.