Perhaps Celestia doesn't not age because she exists outside time, and thus the progression of time is completely irrelevant to her. She knew what was going to happen because, for her, everything has already happened.
In my opinion, I will describe that Princess Vampyrelestia (a.k.a. Princess Vampire Celestia, othe Princess Celestia the Vampiric Alicorn) is actually a photophilic vampire. It means a vampire that never hates light. In short, this is the only vampiric alicorn which is photophilic. [The word othe which means or. It comes from Old English oþþe]
Hmm...so if Celestia is the deity in charge of the sun, does that mean she gets an immunity to sunlight as a vampire? Or will she still get incinerated if she's exposed to it? Would she refuse the raise the sun, and force Luna to do it, thus saving Equestria, but killing her own sister? So many possibilities and questions!
Still, a really interesting idea! Her expression is also quite amusing!