@Background Pony #E016  
Only the most powerful unicorns can pull it off.  
Presumably there aren’t many around who can pull it off, and the ones that can may have more pressing matter to attend than casting that spell on everyone they meet. Like running the country…

@Background Pony #9B14  
So. Why this spell isn’t used as cure-it-all? Why there isn’t young grand-grand-grandparents around?

@Background Pony #E016  
So its youth, without being youth? How would it only selectively rejuvinate parts of the body while leaving others alone? The brain sure isn’t untouched by the spell, and physical muscles aren’t either.  
When snips and snails where hit by the spell, one became an infant incapable of speech, the other so old and frail he could barely stand.  
Seems to me that the spell didn’t just change one part of their bodies, but the whole thing. The state of the brain, health of the eyes and fitness of the muscles included.  
If the spell doesn’t change your effective, healthwise age, why did Snails get so frail when he was aged? His muscles didn’t have time to atrophy the way they do in normal ageing.

@Background Pony #4D7F Milking Twilight you say? An intriguing idea!

@Background Pony #9B14  
And don’t forget about Hayflick limit.

@Background Pony #9B14  
Destiny? Fate? What are you talking about?
 
So, you’re used youth spell. It makes you smaller, voice thinner, etc. But it don’t makes your internal organs miraculously any healther, don’t clear cholesterin from your blood, don’t destroy cancer cells, don’t cure your cumulative diseases. You may look and feel young, but you still will go blind, your liver someday will stop working, etc.

@Background Pony #E016  
So your explanation for why everlasting youth not providing extended lifespans is what? Final destination style rules where death kills you in some imaginative way once a universal clock runs out, due to “Destiny”, “Fate” or some other writers excuse to cover bad writing in a story?

I’m not some Canterlot pansy, I played Academy hoofball, could have gone professional.
 
I could snap Fausticorn in half. With my bare hooves!

@Background Pony #4D7F  
>Being this mad.

@soundtea
 
Or Aliens.

@kircher
 
…I don’t get it.

@Jarkes  
So basically Cadance and Twilight are Uplifted clients…Holy David Brin, Batman!

RULES OF NATUREEEEE!!

@Background Pony #E016  
*before they die of old age

@Mr. Horrible  
I think about it as quantum immortality:
 
Insane theory: this is multiverse, and all universes, where they die, is immediately destroyed by EoH. So only remaining universes has them alive, one way, or another. In the end, before they die, they undergo alicornisation, dracoequusfication, nanomachinification, etc.
 
This is safe, because Multiverse has all possibilities, so Multiverse itself can’t be destroyed.

I have a crazy theory that it wasn’t being alicorns that made Celestia and Luna immortal, it was the Elements of Harmony.
 
It also neatly wraps up that whole “Twilight won’t outlive her friends” thing: All six of them are already immortal and don’t know it yet.

@WinterCutieMark  
I’m saying about The Ageless, not Complete Immortality.  
I don’t know about time spells, but age spells can return youth, but not prolong livespan, otherwise all powerful unicorns and theirs friends will be immortal, not just Celestia and Luna.

@Background Pony #E016  
While that’s true, we don’t actually know for sure that they are immortal. Perhaps eternal youth through ageing or time spells is all they have?

Not the first time I’ve heard that excuse used:  

@DudeManGuy  
[cheering in the background]

Yeah, I’ve always figured that Celestia and Luna, while not necessarily immortal, have much longer lifespans than average ponies. My personal theory is that Celestia and Luna became alicorns in a different way than Cadance and Twilight, or were BORN alicorns, so they have a much longer lifespan. Since Cadance and Twilight ascended, while they will have a longer lifespan than they would have otherwise, it won’t be nearly as long as Celestia and Luna.

@Serris  
If nothing else, that verifies there is indeed, an Alicorn Level.

@Background Pony #9B14  
Ethernal Youth ≠ Immortality.

@PonyPon  
Alicorn level, Trixie only casted it because of the alicorn amulet.
 
So yeah, they could just be casting time magic on themselves.