This rule is absurd, but it has some story behind to explain it.
Today we cannot understand well all the prohibitions of sex around the young people. Not only among adolescents, but to have sex is also rather problematic and becomes difficult and rather improbable in our adult’s life. It looks like the prohibition of sex that started in childhood or adolescence, is following us for the rest of our life till we die. Our sexual problems are not solved even with marriage, they probably increase, and only after our death the problem finish.
I am going to prove why the prohibitions around sex make sense,
It all started after after the farming revolution. Or at least started some 3 or 4 thousand years ago. The reason was excess population and the wars that were followed by population growth.
So, rulers delayed as much as possible the marriage of young people for the said argument, population growth. As it is very rare to hear this argument, I must present some details.
In past times, young people of Athens passed a long time in military encampments training for years. After a time, they served as servants or workers in some industry, or simply as assistants or servants of some people with money. They had not the right to marry unless they own some important business or some important piece of land. The obvious is that among the brothers, only the older first son had a right to inherit the property and to marry. The slaves and in general the servants or workers on some industry had not right to marry for they barely earn to feed themselves. If you have any doubt here are the words of Aristotle, the ideal age for a man to marry is 40 or 43 years of age. A father with a good farm could not be in mood to permit his first-son to marry and have children for he had still problems to feed their children, and paying taxes to the state. 2.500 years ago, problems were similar to the present. He was not eager to make his first-son to marry and make babies, for this would had increased his economic problems. Then it comes to my mind the phrase of Aristotle.
On the other hand women were totally under control at home, to avoid any sexual temptation. Unless her father would found a suitable man with means enough to feed a wife and children, the women would remain chaste for the rest of their life. To marry a daughter was not free either. The father had to segregate a part of his property as dowry to marry a daughter.
A similar picture occurred in Sparta, except for the dowry. Young people passed their life in military campgrounds training for the next war. When they were becoming older as warriors, some selected few were giving the state of some deceased former owner. The rest? They were perhaps servants or assistants of other people, but they probably do not married at all.
Any more doubts? Read the bible. At what age married Jacob? As I do not remember well the details, I googled for it. At what age Jacob married?
At what age did Jacob marry?
ESV Study Bible puts it at 84. Jacob marries Leah and Rachel
Genesis 29:21-30; 30:1, 22-26 A few sites have this logic: > Jacob died when he was 147 years old.
The Life of Jacob
Esau married when he was 40 years old (Gen 26:34). Thus, Jacob also was 40 years old at the time of Gen 26:34.
What about Abraham?
I copy from the Wikipedia,
Abram was 75 years old when he left Haran with his wife Sarai, his nephew Lot, and the substance and souls that they had acquired, and traveled to Shechem in Canaan.
If I understand well, Abraham at 75 years of age, had not yet any children.
There was a severe famine in the land of Canaan, so that Abram and Lot and their households, traveled to Egypt. On the way Abram told Sarai to say that she was his sister, so that the Egyptians would not kill him.
When they entered Egypt, the Pharaoh’s officials praised Sarai’s beauty to Pharaoh, and they took her into the palace and gave Abram goods in exchange. God afflicted Pharaoh and his household with plagues, which led Pharaoh to try to find out what was wrong.
Upon discovering that Sarai was a married woman, Pharaoh demanded that Abram and Sarai to leave.
So far there was not any inheritor of Abraham.
Then there are some narrative about wars once back in Canaan.
Covenant of the pieces
The voice of the Lord came to Abram in a vision and repeated the promise of the land and descendants as numerous as the stars. Abram and God made a covenant ceremony, and God told of the future bondage of Israel in Egypt. God described to Abram the land that his offspring would claim: the land of the Kenites, Kenizzites, Kadmonites, Hittites, Perizzites, Rephaims, Amorites, Canaanites, Girgashites, and Jebusites.
I present this argument to show that existed some problems with breeding not because a genetic problem, but because sex was delayed for a long time. Perhaps several decades.
This happens all the time today. Many women choose to go working to have economic independence and around age 40 they want to have a baby abut the machine does not work anymore. Many of them had to recur to artificial insemination and artificial enhancing of the womb with hormones to make possible to have a baby at this age. But among people of lower class they have not any qualms about copulating and are making a lot of babies.
So it seems that existed in the past some intentional delaying of marriage and thus of of breeding. Not sure if this was a permanent feature of past times, or it was a situation that changed after a great epidemic or something.
But so far, I have guessed that most people in Victorian times were either domestic servants, or badly paid workers in the industry. In any case, industrial salaries in times of Queen Victoria would had not permitted the average worker to marry and raise children, apart from from the need of paying a rent for a diminutive flat. And of course, soldiers and servants do not married at all.
Why the population was growing? Because the families that married were making 7 or 8 children. The rest of the population had not any means to make babies.
But nature asks adolescents to have sex. If we are a bunch of hunter gatherers in the Kalahari or in the paleolithic times, there was not any problem with sex. Why? Young girls were rather thin. Being rather thin meant the ovaries would not be ovulating. The reason? Nature had a safety mechanism so not any women could get pregnant before gaining some amount of fat. This would take a number of years since they start copulating at age 12. Then, the approximate age for a woman to get pregnant in the Kalahari is about 20 or 23 years. But it is not related to lack of sex but to lack of fat. By the time she could had accumulated enough fat to feed the baby in her womb and to breast feed a baby for a couple of years. After storing some fat again she would get pregnant once more. Then, in periods of abundance human population was growing slowly. In the hardest times population was shrinking or barely having a zero growth.