(In my Headcanon, that Celestia give up her immortal for twilight, But the magic that still had it, effect to her age slowly)
She no longer wearing her royal neckless and shoes, just a casual scarf and relaxed.
She also acts like a grandmother who spoiling their grandchildren (Nightfall serenity) and Starbuck Mom. And last, She feels sorrowful seeing nightfall when he visits his mother's funeral.