@Alicorns aging
Um… You do realize that Alicorns Actually continue growing long past the maximum growth of most ponies… Right?
When we saw "Filly" Cadence, that was after her ascension. And she was already slightly bigger than your usual pony. And she continued growing.
Twilight gained a bit of growth after her ascension as well.
And you know where most ponies stop growing? At Regular pony size, the same size as the rest of the Mane Six. Rarity's parents are the same size as her Darn it!
So my Theory is that Twilight will grow to the same size as Cadence, Celestia, Luna, and so on, due in part to becoming an Alicorn.
But the real question here is Immortality, not whether or not they are still growing. The fact they are still growing actually points to the possibility that they are immortal.'
Why?
Let's think about it. Most humans reach their peak growth at about 20 right? Then it's 60 years of decline until death. That's an Age of about 80.
But imagine if once you got to 20, you were still growing? You still hadn't fully matured?
Say it takes till you're 40 to be anywhere near fully grown, Then add to that the 60 or so years needed for the body to fall appart and you're now at 100 year lifespans.
Now let's look at Luna and Celestia.
Celestia is the Oldest Alicorn we know of. Or "Most Mature". With Luna still behind her.
And you'll notice… Luna still hasn't caught up to Celestia. Which at the very least means that it makes a lot more than 1000 years to become a fully grown Alicorn.
I'm going to go out on a limb here and say that although an Alicorn's growth may slow, it doesn't stop. And since it doesn't stop, they've never reached full maturity.
Which means… They can't become "Old", since they never reach their peak.
This however does not mean that Twilight will outlive The others because the others can always find other ways to live as long as her. Whether it be via Age spells or what, it doesn't matter.
There's also this ONE other possibility…
Twilight performs a Heroic Sacrifice!
But do you really want that?