Well, my own thoughts was that Twilight and Cadence won't have eternal life, and will age as normal. They are (from my knowledge) the only ponies we've seen to be small fillies (or teens) eccept for Luna (if I recall) and because we've seen them grow older (unlike Celestia and maybe Luna), they will continue to grow older as time goes by.

(The reason why Luna and Celestia has eternal life may boost my theory why Twilight and Cadence don't have eternal life). Now, you might wonder why Celestia and Luna will have eternal, or, has eternal life. My theory is that because Celestias duty is to raise the sun and Lunas duty is to raise the moon. So, as long as the sun and the moon exist, Celestia and Luna has to live, to bring the sunrise and the dawn.

They MIGHT aslo be seen as the gods of Equestria, wich also, of course, bring eternal life. Cadence isn't a god that bring the sunrise, and Twilight isn't a god that brings the moon. Celestias and Lunas duty to raise the sun and moon is very important for all living things in Equestria, heck, maybe it's very important for everything that lives on THE ENTIRE PLANET (if they live on a planet. I think it isn't comfirmed)!

Because of that (what I know) Twilight and Cadence does not have that very important duties that includes raising the SUN, they don't have eternal life.

And that is my theory why Twilight and Cadence does NOT have eternal life.

Hmm, interesting thoughts. I agree with the theory that Cadence and Twilight are not immortals unlike Celestia and Luna. Perhaps the reason of this is that they have not been born as alicorns, they were converted into alicorns by some sort of magic (possibly Celestia's) It is told in one of the official MLP storybooks that Cadence was originally a pegasus and she seemed to be aging just like Twilight, as we've seen in the Season 2 finale. In that episode we have also seen that Celestia is not invincible at all, which implies that she's not a god. Also, I believe they have confirmed that Twilight does not have eternal life.

However, if Twilight DOES have eternal life, it would mean she'd inevitably have to see her friends grow old and die at some point, which would give space to a much darker story arch not suited for a kids show. Wouldn't such a concept be a little cruel of Celestia, even if she's in pursuit of the greater good of Equestria?

On the other hoof, Lauren Faust once said, that it was her ultimate goal to make Twilight the Princess of the land, which would imply that the days of the original alicorn sisters are numbered in some way, if the writers are planning on following this train of thought, which doesn't seem likely. But who knows, they took the risk of introducing a life-altering change in the most defining aspect of the primary protagonist and the entire status quo of the MLP premise, not to mention how will this alter Twilight's relationship with her freinds, since the symmetry of the mane six have been disrupted...

But as we wait for Season 4 and ahead of an extended period of pony draught, this cliffhanger situation gives us, the fans the space to come of with our own headcanons about this. I have several slightly darker headcanons about Twilicorn, I've even posted one fanfic here, called Alicorn Legacy... :)