I think Celestia and Luna are immortal, or at least very long lived, and that Twilight and Cadance are something else. We've seen Cadance age normally in the series, thanks to Twilight's flashbacks. And I think Twilight is probably simmilar to Cadance. They're alicorns, but they're not imortals like Celestia and Luna.
While Twilight seems to have been destined for this since episode 1, we know that Cadance was roiginaly a unicorn and Hasbro wanted another Princess toy to sell, so she's really just been wedged into the canon of things.
I think they missed an opportunity to foreshadow Twilight by not having Cadance earn her Alicorn status after love beaming Chrysalis. I even think it would have worked a lot better to have done the Crystal Empire as the S2 finale. Have Cadance be another student of Celestia, sent up there to take charge. Earning her wings after she activates the Crystal heart at the end. Heck, they even could have used it to introduce Shining Armor as head of the guard sent to the empire with her, and then do the pony wedding in the S3 premier. Chrysalis attacking a new fledgeling city, rather than Canterlot.