Over ten thousand years old? But according to the show equestria hasn't been around that long and luna and celestia weren't even around as long as equestria was. The show says over one thousand years old so I would base it off that. (Explains luna's immaturity)

2148527 I picked the AU tag specifically so I could play a little fast and loose with the Equestrian lore. If the story were based in the show's universe, you would be absolutely correct, but then there would be much larger issues to address, like a violently psychotic unicorn guardspony, a borderline-sociopath Guard-Captain, and a Celestia that is much too easily flustered by cake than the show suggests she should be.

As for Luna's "immaturity", she's got a coupla character flaws that will be examined later, probably chapter 7 or 8.

Honestly, though, Nix jumps dimensions, and a lot of them are pretty similar to our own world minus a few fucked up hiccups when it comes to monsters/demons/evil gods. How many Equestrias do you think are out there? Nix just happened to land in one that was a bit different from the show version (although nowhere nearly as violent as the Chess Game of the Gods universe, for example).