Re: Everypony will be alicorns...

[Moodyman90]

Don't forget that there were Apple family members there that are older than Granny Smith as well.

[Formis_Fluttergod]

...
Also, nobody ever said that alicorns are immortal. We know only two that lived for few thousands of years and they also control celestial bodies all by themselves.

And nobody said they weren't either.  
All we got from the writing staff is that Twilight is not going to outlive her friends, which can mean several things.
1) Alicorns are not naturally Immortal and Celestia with Luna are an exception (either because of controling the celestial bodies or because of some other yet unrevealed reason).
2) Everypony from Mane 6 will become alicorns.
3) The Elements will try to balance their bearers' life-span in order to increase their efficiency by not having to look for new bearers.
4) Something horrible is going to happen to Twilight that will end her life prematurely, thus not allowing her to outlive her friends.
So many possibilities...

true... can't we approximate how old is she based on the apple family reunion episode? I can't remember if anyone tired that <_<

You might be able to get an approximate time since the first reunion by analyzing the number of squares in the quilt, but since we don't know exact ages of the Mane 6 or of the CMC, we can't know how quickly poneses age, and thus can't get more exact than approximate amounts.

It's obviously because Granny Smith is older than Celestia.

Older than Celestia? Lol, good one.  
Nah, there's no need to bother with the reunions.

From what I remember, Faust said the Mane 6 should be in their teens-adolescence. Considering that Twilight was basically in college, it fits, so that's, let's say, 15-21 human years.

From the Family Appreciation Day we know that Granny Smith was among those who founded Ponyville. At the time she was probably not much older than the Mane 6. From Winter Wrap Up we know that Ponyville's been around for a few hundred years, so Granny Smith is a few hundred years old + some years on top.

There, I solved it for you, it's still just an aproximation, but you won't get much more precise than that.

[Necr0maNceR]

[Moodyman90]

And then they rushed her to the hospital because that's not a horn, that's her brain.

[Formis_Fluttergod]

Derpy:

Wat...

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