Medieval peasants and most tribal cultures...
Typically, children under age 1 were nursed by mothers who nursed them frequently whilst doing other work. Children aged 1-2 might still be nursing, or might already be transitioned to the next group...
Ages 2-4 were supervised by aunts and grandparents, and had as much play as they would ever see; the basic idea was to get them talking, and knowing who to defer to, and be able to take instructions.
Most 4-8 year olds were being tasked with the easier tasks of the family. Girls started helping mom with cooking and gathering in aboriginal societies, and boys doing hunt-imitation type play; both started working the fields with mom in peasant societies. This is where the two begin to diverge sharply.
By age 8-12, most peasants were being tasked to separate tasks, usually supervised by an older sibling or cousin. Play was not well tolerated, and the long days of work were already starting on shaping the adult body. The gender role differences are still minimalized. Note that the local priest might pick a child or two a year, and pull them for life in the church... if boys, and excellent singers, odds are that castration is in their immediate future, but other than that, the life of a cleric of this age is one of constant study, practice, and prayer.
In that same age 8-12 range, aboriginal peoples tend to have the boys starting to accompany the men on lesser hunts, and the girls begin to be serious parts of the gathering team. Girls can look forward to being married shortly after this age bracket, so they are being trained, almost as a commodity, to be good spouses. Boys are taking a slower and longer body building course of increasing effort hunts, and helping with cleaning and carrying prey back.
Ages 13-15 for peasants tend to run to being less supervised and more task focused. The brighter children being given more adult level tasks. Gender role differentiation is really started, and puberty tends to start later in this period; modern 15 year old nubile sexy-shape is mostly absent, due to poor nutrition and lots of physical athleticism (both of which delay both onset of menses and female secondary sexual characteristics). Males tend to hit puberty earlier.
That same ages 13-15 for clerics and religious sees education in writing, many common tasks they already know continue, but at a reduced level (self-sufficiency level, rather than commercial level), and even less tolerance for individuality.
Ages 13-15 for aboriginals generally is considered adulthood for the girls, who are usually married into another tribe or family; they spend this time mostly being incluturated there, and possibly pregnant. Males tend to be assisting in the hunts; while not physically mature, they are still pretty useful, and their contributions are important to the tribe; they are pulling their own weight most of the time.
In any realistic peasant society, a child showing any skill at the occult would either be apprenticed to the local healer, or turned over to the church, or killed. The demon scenario would result in a child killed by one or more of those groups. Murder was a high crime, and a child killing another was usually put to death. Killing livestock was likewise grounds for maiming or death penalties.
In an aboriginal society, where such killings of animals could be easier hidden, the child might stand a chance... or might be cast out. In ether case, if the child shows a magical aptitude, the shaman or shamaness will likely take them from the family at about age 5-8. And if the child shows that much evil, and the shaman finds out, quietly or publicly kill the child, or possibly, just cast them out.
So, for either of these, the most believable is that the child was normal, showed a bit of gift, and then was possessed around age 5-6, and escaped into the wild. A child that age can survive (it's happened several times in the last hundred years in France), and the demon would then have free reign to warp the child.