The Spartan people of Ancient Greece had one of the most unique societal structures that history has ever seen. Although they were never an expansionist power, they were a warrior people who oriented their whole society towards the common goal of turing their male children into the finest warriors in all of Greece. To do that, they devised a harsh period of extensive military training called the agoge. By the time Spartan boys finished the Agoge, they were ready to take on just about any foe.
Spartan boys entered the agoge on their seventh birthday, but they were inspected shortly after birth to make sure they were suitable for it. If any effect was found in them, they were taken to the cliffs of Mt. Taygetos and discarded as too weak to be Spartans. All those who survived began military training at the age of seven, except for the eldest boy of each ruling family.
At seven, Spartan boys were taken from their families and placed in barracks of boys their own age under the leadership of an older boy. They received some education, but the agoge focused more on physical training including martial arts and endurance and strength training. During this period, they were given only enough food to live on and were expected to steal food. They were punished if they were caught, but not for stealing food. They were punished for being caught.
At the age of twelve, Spartan boys were thrown out of the barracks and made to live on their own in the elements. Here they had to overcome hunger, cold, and wild animals. Those who survived returned after a year and entered an even more dangerous period of training consisting mostly of wargames. Many were injured or even died in these wargames, but those who survived graduated from the agoge at the age of twenty with extensive military experience.
Although the Spartan boy had finished the agoge at the age of twenty, that was just the beginning of his service to his city. He lived with the army in the barracks for the next ten years, even if he was married. At thirty, he became a full citizen and could live with his wife and/or enter politics if he wished. He still fought with the army, however, and was subject to being called upon to defend Sparta until he was sixty years old.
The agoge system was successful in producing the finest warriors in Ancient Greece. For hundreds of years, the Spartan army was never defeated when assembled in its full strength. Even when Sparta began its decline, the decline was not the result of military conquest. Rather, Sparta's citizenship laws did not have a naturalization process, so Sparta fell apart as its citizen class slowly died out over a period of hundreds of years.