makes sense to me. They ascended to the position of alicorn by their own ways. They gained a horn or wings, not immorality. plus we have seen Cadence grow up from filly(crystal heart spell) teenager and adult. and Twilight looks to be changing shape also . beside the fact Celestia and Luna are basically gods. They control the sun and the moon. far beyond the powers of any mortal pony.
Thats the type of explanation I have to give whenever somebody thinks they are immortal.
One way you can tell Luna and Tia are powerful is their flowing mane.Have you seen the Episode 1-2 of Season 4?If you have, in the flashback, Luna has the flowing mane.She also had it in the flashback of King Sombras defeat. And that all happened before she was nightmare moon.So my theory is that when the Elements of harmony where used AGAINST her, she was drained from power.Like in the first ever episode of my little pony, she has the normal mane,but in her second appearance, she gets her power back, giving her the flowing mane as well.
Another thing is that if you look closely, you see sparkles of magic around Luna and Tias horn. Which most likely means that they have so much power it leaks out.THE POWER.
Also,if you saw hearths warming eve episode, when Equestria was founded, way long ago, the unicorns brought the sun and moon.So I guess it is possible for a normal pony to raise them, but it must've been very tiring, but the unicorn also did it TOGETHER.Because when Equestria was founded, or was before celestia and Luna.(or B.C,before celestia)
So maybe,just maybe, Twilight bad Cadence are capable of putting up the Moon and Sun.
Btw next episode is called "rarity takes manehatten." And then "Apples to the core."OH OH! And the changelings return for there is confirmed to be 53/63 I forgot, alicorns in season 4.And changelings are Alicorns.
Last, STARSWIRL travels to the future using the time travel spell Twi used.The Equestria goes all nuts and then the parallel universes clash and!OMG!!Im a walking spoiler.Sorry this is EXTREMELY long.