Let's go back in time to when StarSwirl the Bearded was Celestia and Luna's teacher.
Let's start with the fact that according to my personal unsubstantiated assumption, Celestia and Luna were previously unicorns. Here they are depicted after the transformation into alicorns. After all, StarSwirl knew the two sisters already as alicorn princesses.
Also, I don't think that Celestia and Luna's age difference exceeds more than 3-5 years, since, according to my theory, they were ordinary mortal unicorns, but they began to learn from Starswirl at the same time. Maybe Celestia started her training a little earlier.

Here Celeste is still a teenager. Luna is still a colt.
Celestia's mane design is taken from Lauren Faust's early concepts (well, something like that…).
This is the first of four time periods. Next will be Celestia with Twilight and Twilight with Sunset.