In my headcanon, I like to think the Mane Six that aren't Twilight are immortal through one way or another. AJ because of Zapp Apples, Rarity for bathing in well, let's just say it doesn't involve water, Pinkie because she's partly elderitch being from beyond the stars in equine form, or a descendent of Sleipneir, etc.

Shining is immortal because banging an Alicorn gives one immortality

5097866 If the Shining Armor thing was true, there'd be about a billion more immortals running around the shop.

5097961 Well, its a 50/50 chance, and being immortal means you can't die from natural causes, maybe other buck buddies of Celestia and Luna died via other ways

5097866 I point to age spells (as seen in 'Magic Duel')

5097866 Also, most of those came from Who Wants To Live Forever?

5097866 I'm not a fluttercord fan. But Fluttershy can have Discord help her on immortality.

Twilight pulled a 3x3 eyes and bound the other elements lifeforce to her. meaning they can't die because she's immortal

Headcanon-wise, I think that currently, the Mane 6 might well be immortal/ageless, because of the Rainbow Power (and Twilight for obvious reasons). Shining Armor probably isn't - but might end up being the first alicorn stallion. They never establish that the Rainbow Power makes you ageless, but Word of God says Twilight won't outlive her friends, and I suspect that means that they'll all live indefinitely. (Of course, they might all get promoted to alicornhood, I could see that, but...)