First off I haven't seen the movies but I do have a basic understanding
1) Parallel universe
2) Same characters as humans
So what I noticed is that all the characters appear to be the same age as their counterparts.
When Twilight goes there she turns into a human and is the same age as the "Already existing" Mane 6 in that universe
So this tells me that when say Pony Twilight was born Human Twilight was also born the universes are synced in that regard. When X Character is born in one reality they're also born in the other reality
So what about Celestia and Luna
Clearly they exists in the other reality as "adults" but in Equestria they're thousands of years old. By all accounts if I'm right Human Celestia and Human Luna were born at the same times a Pony Celestia and Pony Luna, meaning the human versions are at least a thousand years old or more.
The movie shows that there is magic in the "Human universe" but it works differently so what are Celestia and Luna in the Human universe because they're clearly not normal humans by that universes standards.