Talk:Princess Luna/@comment-25975201-20150130003116/@comment-10390252-20150130145119

FWIW, my headcanon is that Celestia and Luna were both young adults during Discord's reign over Equestria. Arbitarily, I have put that at 1,500 years before the present day (half-way in between the founding of Equestria by Platinum, Hurricane, Puddinghead, Clover, Pansy and Smart Cookie and the fall of Sombra). In the show canon, we don't really know when it was other than it was before the disappearance of the Crystal Empire 'about 1,000 years ago' (read: 'Once upon a time...').

The chapter book Twilight Sparkle and the Crystal Heart Spell says that Celestia and Luna were young adults at the time Star Swirl the Bearded approached them to lead Equestria after the unification of the three tribes. However, once again, no firm time period was given for that other than it was evidently some time before Discord's reign. However, the chapter books and the show are considered parallel continuities rather than necessarily the same canon; take it as seriously and as authoritatively as you wish.

No matter how far back it was, I'm fairly sure that there are only a few years' difference in age between them.