So Powerful Mages Do Live Longer · 3:54pm Sep 23rd, 2017
Seems this finale is holding to the standard the show has been using pretty much since the start. Powerful enough magic users are resistant to age. This means Twilight was most likely going to outlive her friends long before she became a princess. Most likely she was going to outlive the average pony ever since she got her cutie mark given the raw amount of magic she unleashed demonstrating her powers to be more than enough to qualify.
Well, AJ could be another Granny Smith and live a few centuries herself. In fact, if you take some of the information at face value, Granny would have been much more fit if it weren't for her hip injury.
Same for the others. Heck, maybe becoming the Bearers comes with benefits! Or the Rainbow Power, or any number of other factors.
Pinkie is always Eternal, though.
4676344
Since they all host the Rainbow Power, I agree that they'll all live longer than average now. AJ I expect to live at least two centuries since her grandmother has at least done the same. As for Pinkie... well I consider her an eldritch abomination so of course she is eternal. Assuming she isn't killed somehow through diabetes anyway.
4676403
She just transcends her physical body and becomes pure
energymagicparty power.Also, her kids spread far across the globe and manage to cartoon-shenanigan it into a higher plane of existence as well.