What is the best age to start training young children for warfare by an elite military?

Question

Note: this is NOT a duplicate of this question as that question asks whether the process will create better soldiers or not, while in my setting it is already accepted by the empire that it WILL create better soldiers. Also, the question is asking more about a modern setting.

In my (medieval, magic, pre-gunpowder) setting, families, especially poor families, in a certain Empire can give up their child to Imperial Military Orphanages in exchange for a lot of money and Imperial residences in specific cases. The families earn a lot of prestige if their child does well (in case they choose to reveal themselves; a lot of parents choose not to tell their children who they are, that way they gain the money and prestige of being child donors while not having to deal with a child who hates them since he was discarded.) Noblemen often give up secondary and later male children to disinherit them and avoid succession disputes. Children are also confiscated as punishment.

These children are taken very young; many are not even weaned and so the Empire employs many wet nurses for this purposes. Around 2 - 18 months old, the average being 8-9 month old children. (Younger children, but those that HAVE been weaned, pay more than older ones. 8-9 months is the most profitable.)

Obviously you can't start training 1 year old for military. Once registered at orphanages, these children are sent to foster families that have 6-7 children (all foster) each for mental development and teaching of basic language and arithmetic skills, until they reach a specific age at which they are sent for long periods of time to orphanages-cum-training centers, where they stay till they are around 16-18 and are trained to join the Empire's elite forces. They see their foster families only a few times a year, and, depending on what their biological parents chose, may never see them again.

But what is the best age at which they should be sent to military orphanages? How young or old should the Empire start training these children to create the best, most indoctrinated, yet still functioning human soldiers?

The only historical estimate I know of is 5 years of age, from the fact that 5-8 was the age children in ancient India were sent to Gurukuls, but that is still radically different as the only purpose of this program is to create war machines, and though they are taught arithmetic, language, reading and writing, history, religion and magical theory, there is a great focus on combat skills and training with only magic (and to some extent religion) being additional subjects with more than cursory focus to them.

Answer 1

Never heard of Sparta agōgē? (emphasis mine)

The agōgē (Greek: ἀγωγή in Attic Greek, or ἀγωγά, agōgā in Doric Greek) was the rigorous education and training program mandated for all male Spartan citizens, except for the firstborn son in the ruling houses, Eurypontid and Agiad. The training involved cultivating loyalty to the Spartan group, military training (e.g., pain tolerance), hunting, dancing, singing, and social (communicating) preparation. The word agōgē had various meaning in ancient Greek, but in this context it generally meant leading, guidance, or training.

According to folklore, agōgē was introduced by the semi-mythical Spartan law-giver Lycurgus but its origins are thought to be between the 7th and 6th centuries BC when the state trained male citizens from the ages of seven to twenty-one.

The aim of the system was to produce strong and capable warriors to serve the Spartan army. It encouraged conformity and the importance of the Spartan state over one's personal interest and generated the future elites of Sparta. The men would become the "walls of Sparta" because Sparta was the only Greek city with no defensive walls after they had been demolished at the order of Lycurgus. Discipline was strict and the males were encouraged to fight amongst themselves to determine the strongest member of the group.

If we have to believe all what traditions have told us, it was pretty effective at making good warriors.

Enforcing it into an empire and only on a subset of all the children would probably mitigate the drawback it had in the small reality of Sparta: having to rely on large masses of slaves for doing anything which wasn't training for war, denying citizenship to those who failed it, tend to have long term negative effects on a small city, which is probably why we never heard of a Spartan empire.

Answer 2

Four to Six, kind of

Any younger and you don't have a soldier-in-training, you have a head case. Plus they're more likely to die of all the awful infant/toddler diseases and waste the Emperor's valuable coin! Even in a medieval setting you can't just pluck a 4 year old out his home and train him 24/7 to be a soldier. Four is my lower limit, because that's about how old a child needs to be to sit a pony on their own. So at four if you want "born in the saddle" good cavalry that's where you need to begin. I choose an upper limit of six because, traditionally, that's the age you need to start shooting arrows if you want to become a truly skilled longbowmen. If your kids need to learn neither archery nor riding the time may vary, but what sort of elite medieval soldier can't ride OR shoot??

The thing is, they CANNOT be taught only "fighting things" at either age. It basically needs to be a daycare that offers pony rides and archery lessons. Why? Because you NEED your kids to grow up with the normal range of human emotions to function well as soldiers. On top of that, you need to teach them TONS of stuff. Math, writing, philosophy, geography, agriculture, metallurgy, engineering... all will make them better soldiers. The problem with Sparta (And BOY did the Spartans have great PR and so-so-at-best training. for more see THIS but sparknotes is the agōgē was garbage, spartans were medicore-soldiers-at-best, and no small part of that is due to their only "training" being fighting.

So at what point should your Jannisaries make the jump from this to full-on "Military Academy"? I'd say around 16. As a kid grows in size and strength you can include more and more military rigors into their life (basic swordsmanship at 10, formation drill at 8, etc) but once puberty kicks in your pretty safe in turning up the military aspect. Though I would still include all the non-military training as well because as Thyucididies said "the nation that makes great distinction between its scholars and its warriors will have its thinking done by cowards and its fighting done by fools."

I'd also have some sort of "off-ramp" for children that don't show military aptitude by the time they hit puberty. A terminally uncoordinated kid shouldn't be forced to be a bad soldier/killed when they could just as easily be shunted sideways into the Imperial Accountants office or what have you. Waste not want not!

Answer 3

Ottoman Empire's Devshirme system forcibly recruited male children from the Balkan christian families under its control. The child had to be healthy, physically fit with no scars, and the child had to have brothers. Christian families only one son, jews, gypsies and muslims were exempt.

The children were selected at the age interval of 8-20. They were not recruited young as strong family values were expected from the recruited children. They were all given proper education and training. The elite of the elite, brighter ones were educated at the palace schools and became high ranking Pashas and even Grand Vizier.

Not recruiting too young had its advantages. You didn't need to find foster families, the children had family values, as they also knew the situation of peasants like their families, the children were aware of how lucky they are to be a part of an elite, valuable, well paid part of society. So they were loyal to the sultan (for a few centuries at least heheh).