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Hokay, trying my hand at this once again.

When I first embarked on this theory, it contained rampant extrapolation, unlikely mathematics, and incredible feats of suppositional derring-do; but this version is much more simple, and since it also actually makes sense now, I'm going to say that Occam is probably correct this time around.

I believe that the koorime are only supposed to have one child in their lifetime.

I had denounced this before on the basis of phrasing -- "Every 100 years, to each koorime a child is born." -- but then I realized that my fail was inherent in the fact that this phrasing is in English. *facepalm* Granted, it's a lovely dub, but one line that's supposed to sound grand and archaic and is probably not grammatically or connotationally the same in Japanese is no basis for throwing out an entirely valid theory. So, since there was no more reason to chuck it, I explored it and found answers.

Consider that Yukina had to run away from the island to look for Hiei, because they would never have allowed her to leave otherwise. Hina, too, was denied the chance to leave the island and live elsewhere so that Hiei's life would be spared. A third point to note is the extreme rarity of the koorime in the demon world at large; if they were freely allowed to leave and live elsewhere, they would be much less rare. A social taboo on leaving is one of the hallmarks of not only a rigid society, but one with a low birth rate or otherwise unstable population. If the koorime only have one child each in their lifetimes, that neatly explains why leaving is discouraged at all costs, and why it was implied that the koorime allowed Yukina to return, even briefly, after she was rescued. Consider as well the rampant over-population that would otherwise result from a peaceful, war-free, almost-totally-safe society that lives on a totally isolated, not-even-connected-to-the-ground floating island. Even two children each would be too many, and eventually spill over the island's sustainable resources.

Now, the series states that the koorime have lived on the island, with their culture preserved, for millennia -- to the point where there have been other Forbidden Children, albeit few. To an extent, the size of the island is therefore a determining factor; with a one-to-one replacement ratio, the maximum number of women the island can support is equal to the maximum number of years the koorime can live. If, for example, the island only supports 500 at any given time, then the lifespan has to cap out at 500 years. This keeps the population stable. As an example, I'll use this 500-year lifespan premise:

000 years: 100 koorime --> first generation are infants
100 years: 200 koorime --> first generation gives birth
200 years: 300 koorime --> second generation gives birth
300 years: 400 koorime --> third generation gives birth
400 years: 500 koorime --> fourth generation gives birth
500 years: 500 koorime --> fifth generation gives birth, first generation dies
600 years: 500 koorime --> sixth generation gives birth, second generation dies
700 years: 500 koorime --> seventh generation gives birth, third generation dies
. . . et cetera.

Just for comparison, if the koorime were to have just two children each, it would be a much different scenario:

000 years: 100 koorime --> first generation are infants
100 years: 200 koorime --> first generation gives birth
200 years: 400 koorime --> first and second generations give birth
300 years: 600 koorime --> second and third generations give birth
400 years: 800 koorime --> third and fourth generations give birth
500 years: 900 koorime --> fourth and fifth generations give birth, first generation dies
600 years: 1000 koorime --> fifth and sixth generations give birth, second generation dies
700 years: 1100 koorime --> sixth and seventh generations give birth, third generation dies
. . . et cetera.

Obviously, this second scenario rapidly overflows the island in terms of sheer space constraint, so that even if they foraged for supplies elsewhere, they would be packed like sardines in a can. The one-to-one ratio is the only thing that keeps them stable. Since we're not given enough context for the island in general to make an accurate guess as to its population capacity (especially since we don't know how much of it is livable or what kind of natural resources it has), this information only really lets us put an absolute upper cap on it of 1000. As shown in the anime, at the very least, the island cannot possibly be bigger than that, and that's assuming all of it is directly habitable.

Moving on, consider also that Hina was visibly an adult in the flashbacks -- not an adolescent just into puberty, but a full adult woman. Hiei and Yukina, therefore, are nowhere near reproductively mature. Since the koorime usually have children without fertilization, this means that whatever age Hina was, is the age when koorime breed. Given the 100 year tag, it's logical to assume this means she was 100 years old, and although this could be mis-phrasing again, I'm going to work with that.

Here's where it gets all tricky and mathematical. Even though we can generalize and say for certain that Hiei and Yukina are younger than 100, it's impossible to pin an actual number on them without knowing the approximate lifespan of their people. We can extrapolate from that the rate of physical aging and compare it to their appearance. They both look somewhere around fourteen at the most, and I'd hazard younger since Yuusuke looks older than they do, so I place them at equivalent to human age 12, while Hina appears to be close to 25. While Hiei's deep voice doesn't really sound like he's the equivalent of twelve, and he's certainly not built like he is, both of these can be relatively easily accounted for. The voice could be a peculiarity of demons, or the fact that he's an abberation; the muscled physique is because he's had years to train his body. He might not be mature as his race measures it, but years of training is still years of training. Yukina appears that age in all ways, having undergone no such training and possessing a naturally higher voice.

Due to this, I place their people's lifespan at 375 years, and their age at 50.

These numbers may seem arbitrary, but they're not. When you double 12, you get 24, close to Hina's apparent age. If the koorime breed at that age, then Hiei and Yukina must necessarily be about halfway to breeding age themselves. The breeding age is 100, so that makes them 50. Additionally, age 12 is 13.333% of a 90-year human lifespan. 13.333% of 375 is exactly 50. If we assume a steady relative rate of maturation and aging, this placement fits all requirements: that the overall lifespan must be sustainable for a small, isolated island; that said island's sustainable population cannot reasonably be over 1000; that their age must be below 100; that their rate of aging must at least somewhat compare to their mother's; that their rate of maturation must be consistent with their race's lifespan. I know that 375 is an astonishingly short life expectancy when compared to the various 1000-and-still-in-their-prime demons the show throws around, but no one ever said every race had to be the same.

So not only does this particular theory pan out mathematically as well as logically, it makes far fewer assumptions than its predecessor. Granted, it all goes out the window if the koorime employ artificial population control, but in the absence of evidence, I'm going to run with the idea that they don't. This is mostly because if they do, depending on what they do (which could be anything from an egregiously long list), their lifespan and therefore Hiei's and Yukina's age could be damned near anything. I work with what I have.

Somebody debate me on this. I'm totally hoping for it.

Yes, I tried my best to acknowledge that artificial population control nixes my entire theory. But it's not possible to formulate any theory at all without either assuming that they don't do that, or knowing what it is they do.

That said, glad you liked it. ^^ I worked hard.

As for them being twins but half-siblings . . . Hiei was so lying through his teeth. It wasn't just that he said they were half-siblings, he said they had different mothers, which is clearly a fib in light of his backstory in general. I'm inclined to assume he just wanted Yuusuke off his case when he said that.

Good work.

I respect your theory completely, and would not be able to contradict it without the evidence provided.

You claimed Hina appeared to be 25.

But in my opinion, she looked pretty young (especially for her age lol). I would place her more around 20.

So if they grow at a rate of .2%, If Hiei and Yukina appeared 15, they would be 75. That works, I believe.

As HIEILUVER stated, I don't think the Koorime have trouble with pushing people off cliffs. If it got overpopulated, they would send people sky diving...without parachutes.

I think the people of Koorime are naturally smallish. I mean, look at the comic. You don't see any of them exactly hitting six foot anytime soon.

It also says the Koorime life span is "nearly endless." Their demons. Their growth rate isn't necessarily constant.

I mean, look at Yumi's kid (forgot his name, Shiori maybe.) He grew from toddler to like, a ten year old in 200 days.

You're theory was very well researched, but I thought I might just bring a few more objects to the table.

Lol. OMG thank you sooo much! I've been trying to remember his name FOREVER! lol Shiori? I'm such an idiot. XD

Yeah I completely see your point. I'm in denial though. I don't want my Hiei to be 12. o_o;;; *goes into dark corner and starts hugging imaginary Hiei plushie*

Me: It's ok, she doesn't mean it. I know you're real age. We'll be together forever. *insert maniacal laughter here* O_o;;


Anywho. I do have to admit. Yukina and Hiei are VERY smallish. They could TOTALLY pass as 12. Plus, it says Hiei's only 4'11 which is a...semi-average height for a 12 year old.

Thank you sooo much btw. All you're posts have been so helpful to my fan fiction! My OC will be learning demon anatomy, which I'll have to make up!

Indeed; I'm not basing my age estimation off of their height, but mostly off of the contrast between their relative youthfulness (using facial features as a base) and Yuusuke's -- also Keiko's, and anyone else whose age we know for sure. And, yes, if my estimation is right, Hiei would be 50. He'd just be the equivalent of twelve for his race's lifespan. Ergo, he's not physically mature, but he's still been alive for 50 years, and is quite old enough to be considered an adult by mental/emotional standards.

And even if they're closer to 14 or so, it wouldn't affect the math much. 12 is just my best guess AND what works best for the math. ^^