Here is “older” Celestia is now retired alicorn. And happily living the rest of her days in peace and quiet at Silver Shoals with her little sister, Luna. And I imagine Celestia design would look like when she “older” and Her mane color almost faded by her age.
 
(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.