Living and Dying: the Cycle of the Soul

Here are the basics:

• Anything that is living, from bacteria to beluga whales, has a soul.
• All souls have two components at birth: life energy, and death energy.
• Over one's life, life energy is consumed and changed into death energy, culminating in the death of the soul.

Generally, all creatures are born with a similar smidge of death energy in their soul, with the amount of life energy then determining how long they will live, assuming there is no outside meddling. Bodies born without one or the other cannot subsist on their own (stillborn). In this sense, there is no creature without expiration: though some causes may not be from aging, no being is immortal.
The more equal the ratio of life to death energies at birth, the faster the body will age and die. The greater the amount of life energy, the longer the body will be at peak health; though the soul does not determine the precise fitness of the person, those of less energy will peak earlier in youth and decay in how fit they may be more soon than others. In this sense, the soul sets the peak: while the soul may still provide youth into adulthood, habits of the body can effectively prevent the maintenance of that youthful state. Conversely, a low amount of life energy may in many cases hinder the body, the imbalance as cause for ill health despite one's best efforts.
Though bodily aging may speed the decay of the soul, it is nearly never to decay it so much as to shorten one's lifespan. There are several ways to die at the same age, and while the soul sets the age, it does not determine the path. While fatal bodily damage (severe burns, suffocation, accidents) may appear to contradict this, there are few cases where this is truly exception.

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True death from oneself generally classifies, ignoring those where ill habits cause. Fate predicts patterns of decline, but is imperfect in the cases of intent and suicide.

tw: suicide
At death, the soul and body separate, the soul joining all others with Death, then entirely composed of death energy1. For sentient creatures2, the soul will retain the memories of the creature, effectively maintaining it after death; for any weaker beings, the energy is indistinguishable from any other, in a sense an "unmarked" form. It should be noted that "weaker" beings does not imply strength of the soul: though longevity and strength are connected, and most sentient beings have "stronger" souls (i.e having more energy), there is no correlation or causation between strength of the soul and sentience or intelligence.
That all said, there's plenty more that can happen when you meddle with the soul, the subject of the next page, Manipulation and Transformation.

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1 "entirely composed of death energy": in cases where the soul has been manipulated, this may vary. See: Manipulation and Transformation
2 "sentient creatures": in fiction, this best correlates to creativity, although it appears to be a mix of different factors that heavily discriminates based on lifespan, social abilities, and memory. Luckily, this means there are no sharks in the afterlife, but there are crows.