Every so often, an earth pony or unicorn is born with a red spot.

At age 12, the spot turns green.

At 25, the spot turns a deep blue.

at 45, the spot turns black and does not change further.

this spot, from birth, is the sign that the pony will never die of old age, for they are a Struldbug. an Immortal.

Struldbug is a word from the island of Luggnag, where this phenomenon is common, with about 74 currently living there.

when they reach 80, they have the infirmities of age. they become opinionative, peevish, covetous, morose, vain, talkative, incapable of friendship, and have no affection for any family after their grandchildren.

On Luggnagg, several laws are applied to them. As soon as they have completed the term of eighty years, they are looked on as dead in law; their heirs immediately succeed to their estates; only a small pittance is reserved for their support; and the poor ones are maintained at the public charge. After that period, they are held incapable of any employment of trust or profit; they cannot purchase lands, or take leases; neither are they allowed to be witnesses in any cause, either civil or criminal, not even for the decision of meers (metes) and bounds.

They never remember anything except what they learned in their youth and middle age, and even that is imperfect. the least miserable are those who turn to dotage, and entirely lose their memories. at 90, they lose the last of their teeth and mane, as well as their sense of taste. what diseases they have never grow better or worse. when talking, they forget names and common appellations, and by this same virtue, cannot read, as their memory cannot carry them to the end of a sentence.

and they continue like this. Never dying of old age or sickness.

2718115 Yep.

This is why we know alicorns are not immortal