What special attributes would a specific age range possess making it preferrable for exsanguination and life force transference?

Question

The blood rite is an ancient ritual in which a bruja steals the life force of an individual and adds it to her own to extend her life. The bruja places the victim inside a cauldron, in which they are murdered by having their throat slit. They exsanguinate their victim - the blood used to fill up the cauldron. The bruja then enters the cauldron with the body and bathes in the blood of the sacrifice as it boils to 100 degrees Fahrenheit due to the ritual.

As the ritual adds the life force of the sacrifice to the bruja, it is logically concluded that the most efficient sacrifices would be children. As they are the youngest victim's, they would hold the most life force compared to others. However, it has been determined that the best sacrifices to use would be individuals ranging from 25 to 49 years old. This age range is referred to as the "Goldilocks Zone", which are normally the best candidates to use as sacrifices for the ritual.

What would be special about this age range that would make it more valuable than others?

Answer 1

This would be the age range in which humans could have the highest base fitness. It is commonly believed people still develop their bodies until the age of 25. Afterwards, the body stops developing and slowly deteriorates. However the largest parts of deterioration aren't too significant until the age of 50, after which people become slower and less adaptable, and have less energy. Midlife crisis happens because people realise they aren't who they used to be before.

In short, the age of 25-49 is what is commonly referred to as "the prime of your life".

This means several things. First, this means the individuals mental state is most developed. Their mental fortitude could be contributing to what you refer to as "Life force", they consider themselves having the most life. If this is related to magical effects, this could be an explanation the blood is magically imbued with their mental fortitude.

Secondly, this is the age range an individual commonly has the best developed antibodies. All diseases a child can have, most adults of this age are immune to. Either due to vaccination or natural immunity development, or both. Or, more gruesomely, natural selection. However after this age this immunity slowly gets compromised as the blood gets "polluted" with a whole new set of diseases plaguing the elderly. For example cancer, blood clots, arthritis, you name it.

Third, children don't have that much blood, so a larger amount of children is needed. Even if the children have more mental fortitude, the amount of blood they poses is simply not enough to get the same effect.

Another option could be that life force isn't just related to time left to live but also time lived. This can be optional in a few ways:

Related to my first option above, the mental state improves with knowledge and experience, and the ego grows. They consider themselves stronger due to their experience, and count their chances of survival higher due to this.

Another way would be that the soul develops like any other organ. It grows and develops with the individual, and is most developed at this age.

Answer 2

This age range has the best blood.

Children and the elderly are plagued with disease. Since ancient times, these vulnerable populations have been avoided by brujas because sickness reduces life force and threatens to break the spell. Granted, these populations today may not be dying of the plague, but tradition is tradition, and the old spells work best.

Life force is also directly proportional to the amount of blood an individual has. Fully-grown adults may resist more, but once killed, they will provide the most blood. For a costly ritual, you'll want the most bang for your buck.

Answer 3

It is the Goldilocks Zone for life theft because of multiple contributing factors. Each one is not the singular defining factor, but have their own ideal age ranges of effectiveness. But by putting all the factors together, it winds up that the age range of 25 to 49 ends up to being the best likely return on sacrificial investment.

I could see an urban fantasy situation where somebody carefully calculates various factors to try to find the ideal sacrifices for their rituals.

But for this rather dark riutal, there are a few factors to take into account, such as:

Development and Immunity

This one has been touched on already. In short, children are developing for years, and the elderly have failing bodies and processes. The ritual will work better the healthier the body is, which tends to favour those in the 25 to 49 range. They are fully developed without the potential plethora of medical issues that older folks can develop. In addition, their immune systems are likely uncompromised so they will not be likely to fall sick in the time between acquisition and sacrifice.

Fertility

The ritual steals life, but it is not just the life that is that is taken, but the lives that could be. Those potential lives are not as valuable as the lives that could have been, but a million slivers of life can add up to an appreciable amount.

Children are not fertile as a general rule. And while the human female is born with all the reproductive potential that they have, those slivers of life are not awakened until puberty. Only once the potential for them to be life is unlocked do those shards of life awaken. Likewise, the elderly have lower or no fertility, with humans being one of the few animals to go through menopause. As such, by the same logic that works for children, an elderly woman won't have those shards of life as they have gone dormant again so there is less potential to steal.

Like the previous section, while those in their teens and low twenties have that extra fertility life, it is those in this range that have a more potent fertility, though this metric does favour those in the younger side of the range.

Return and Compatibility

Logically, to steal life there must be life to steal. In theory children have the longest lifespan to take, but there are factors that limit the bruja's ability to take and harness that life.

But while we think of lifespan as a measure of the overall population, the lifespan that is stolen is tied to the victim itself and what they would have had left had they not been ritually sacrificed. Young children might have the most potential life, but they might have also been destined to die young so that there is a large gap between expected and actual returns. At the other end, elderly folks have a lesser difference in this regard but also less overall life remaining. As for the 25 to 49 demographic, they have more life to steal than the elderly and less variance in remaining time than children, making them a good balance of potential.

The second big factor in the theft is compatibility. How close is the life that the bruja is stealing to their own lifeforce. The farther from the lifeforce that the bruja had when they first undertook this dark ritual, then the more stolen life that is needed to make what is left theirs. Since this is most likely used to stave off death, the bruja is more likely to be in the upper range of the 25 to 49 age group.

Availability

Let's face it, unless there is something seriously sketchy going on, then a bruja is not going to be able to go to the local Kids Mart, and pick up a kid for their sacrifice later. Many parents are protective of their kids, and take steps to make sure they aren't kidnapped by unscrupulous casters. Likewise as people get older, many move around less and stay home more. Some will be taken care of by younger and more abled adult children, which double as protection for them. This limits a bruja's access to these age groups

While exceptions exist like beggar children on streets and older travelling monks, the above is the general case. Adults are just more available and if careful, get into accidents enough that if one disappears in the woods one day, it can be assumed that they died out there or fell victim to random chance. An travelling merchant is more likely to be taken by bandits and killed as opposed to kidnapped and sacrificed to dark rituals.

Admittedly this is not strictly a biological reason, but it does affect the readily available supply.

Answer 4

Lymphocytes! Prime age humans have fully developed and functional immune system, children have not fully developed immune system yet, and senior age humans have immune system slowly fading. And because of dark arts and quite unhealthy rituals/ingredients they consume, bruja's immune system is usually quite compromised. Also its worth notice, that blood is not actually boiling in cauldron, its all smoke and mirrors, because high temperatures coagulate and destroy blood cells and nutrients.

Answer 5

That sounds pretty close to peak fertile age.

Which is when the victim's native magic (all life forms have native magic) is tuned to the creation of new life rather than the protection of its own. This means it takes a much smaller magical shock to make the magic support another existing life (the bruja's).

Answer 6

Life force is not the same as time.

Babies have a lot of time, but not a lot of life force. They're new to the world and life has wrapped them just recently. They still need to grow, to strengthen. Life imparts a force on reality, and reality pushes back on life.
Humans go from a stage of growth (childhood) to a stage of metamorphosis (adolescence).

Their life is too much in flux to be pushing on anything with much force at all. Life has to settle and cure, like concrete, before it can do much pushing. Our steel is blunt, and requires the whetstone of adolescence.

At 25 people have settled mostly, and they're rapidly approaching the bell curve of life force. People listen to what we have to say, taking our opinions with a bit more clout. Our physical exertion turns into muscle more readily, because our bodies are part of reality. We have grand ambitions, to see monuments built for the sake of all. Our steel is sharp, but we don't yet know fully how best to use it.

At 37, the halfway point, people will on average reach the peak of force they can exert. Not physically, but in how strongly our lives can afflict themselves on the reality around us. It's not merely pushing a heavy rock, this force is culmination of willpower, physical aptitude, and emotional confidence that bends reality to suite our needs. Our steel is a deadly weapon.

By 50 we are on the way down and out. We don't just get physically weaker, necessarily, but our drive to impart change on the world is diminished. Apathy, the symptom of a lack of life force, begins to set in. We hold less sway over people, getting favoring votes is harder than ever. We bruise more easily, and strain more readily. Our steel is no longer sharp.

In old age we grow happy to leave the world to the younger generation, ambition drained from us. Depression is more commonplace as the harshness of reality has eroded our life force like water through rock. Our steel is blunted.

(This isn't a commentary on reality, but on what symptoms could be part of OP's fantasy world. These categories clearly don't hold true at all for the real world.)

Answer 7

The life force comes from the conciousness/willpower

Babies and children, though having potential, don't have life force, aka life experiences, a fully developed mind, complex thoughts and feelings. Old age start wearing and taxing the mind (more if they come from medieval selfdom), escaping gradually from the goldilocks zone.

This also serves as justification for not using an unlimited amount of easely avaiable animals, such as a cauldron full of rat blood, or simply hunted game used for food.

Answer 8

The age range of 25 to 49 is the optimal health range for adult human beings. It is a time of life before the effects of ageing start to make their appreance. It may be reasonably assumed that this is also the same age range for optimal strength of the life force. It is likely to be the case that a person's life force needs time to mature and develop to its fullest strength.

While children may appear to be full of life force, this life force is still immature and not at the most effective strength for a bruja to absorb sufficient of it to extend their life.

Adults in the specified age range deliver both the best quality and quantity of life force for a bruja. Why would they settle for less?

Answer 9

This is the age range where humans reach their peak body development and have the best body repair systems. The ritual could work by utilizing this repair system to restore the body of the bruja by activating the immune system of the sacrifice to previously unknown levels of activity.

Answer 10

If a disease or similar can be detected in a blood test, I would argue that it is of "less pure" quality. As such, i would consider the younger the "doner" the better in terms of quality of blood is.

Key factors in blood quality

1. The kidney’s ability to excrete waste products decreases with age Cholesterol levels tend to rise with age, making cardiovascular disease more likely.
2. Use (or misuse) of substances such as caffeine, alcohol and drugs are more likely in older "doners"
3. During puberty a number of changes to blood composition are introduced, for example the increase in Estrogen/testosterone
4. The older the "doner" is, the more likely they are to have suffered various diseases over time, from chicken pox to cancers. These leave detectable markers in the blood.

However, it may be these reductions in blood quality that make the blood more effective in this scenario. A higher Cholesterol, for example, may mean that the blood continues to provide lifeforce over an extended period while the "pure" blood burnt out.