Well, simply put, the modern image of child soldiers in Africa (as in young boys and girls, kidnapped from villages, armed with AKs and indoctrinated into a movement) is something that only really emerged during the late 1980s and early 1990s. There is no evidence that pre-colonial African societies employed child soldiers to any greater extent, and in most cases they did so to a much lesser extent, than was common within European, Asian, and Arabic societies during the same period (consider Italian infanteria or Middle Age squires). However, making any conclusive statements regarding the use of child soldiers in Africa's history presents some big problems.

If we take the 2003 UN definition of a child soldier as anybody under 18 (as opposed to the 1949 Geneva Convention addition that set it at 15) who acts as a combatant, porter, cook, servant, or provides a sexual service, then African history has seen some examples of children acting in a military capacity during both the pre-colonial, and to a much lesser extent, colonial eras. But children and the idea of a 'child' is a loose concept throughout history (maybe not to the extent that societies didn't recognize a child as being different from an adult, as some historians have argued) but certainly the use of children in such roles may not have been noted as being extraordinary by pre-colonial African societies. The modern definition of childhood, as noted above, places children as an exclusive protected category to an extent that they would not necessarily have been previously. That is not to say that children were treated like adults, far from it as I'll show later, but the legal definitions (putting an age on childhood) presents some problems and ends up being quite anachronistic and undermines what we are trying to find in older African sources - just because the sources don't specifically mention the use of child soldiers doesn't mean they weren't there.

And yet there are examples of the outright refusal to use child soldiers in African history. One example I have seen used in the past to disprove the notion of child soldiers in Africa as being explained away as part of their history, is the infamous Shaka Zulu and his period of military campaigning during the early nineteenth century. The Zulu, building on the Tswana speaking Nguni peoples history of age sects or mephato, militarised their social and political structures around the amabutho or age regiment systems, where men would undergo an initiation ceremony as a rite of passage into manhood, bonded with similarly aged men for life, given a regimental name, and placed under the leadership of experienced (mostly) generals called indunas. In such a case you would assume that Shaka and other Nguni leaders such as Dingiswayo, leader of the Mtetwa, and Zwide, the leader of the Ndwandwe, would try to form these amabutho from as young an age as possible. And yet they didn't, preferring instead to wait until the men were old enough, around 15 years old, before permitting them to join the military regiments. Consequently, if you go by the 1949 age restriction of 15, they weren't technically child soldiers. However, if you class children as being 18 or under, then they certainly were child soldiers. There are other examples such as the Kano region of West Africa where children were regarded as being outside of the viable military targets, but I don't really know too much about the pre-colonial history of that region.

So if child soldiers in Africa do not have a long historical tradition, why do we suddenly find the continent inundated with them from the mid-1980s onwards? I think we must consider the post-colonial nature of African states, and the economic and political turmoil that has seen child labour rise along with the number of Coup d'états. The problems facing children in the war-torn countries where child soldiers were most often seen (Sierra Leone, Cambodia, Mozambique, Angola, Uganda etc) can be tied into the value of child labour or rather children as an important part of the economic population. The change in the predominant style of warfare since the end of European imperialism in Africa also helps explain the growing use of child soldiers - the shift from what could be called 'national' or 'international' conflicts between states, developed during the wars for independence of the 1960s and after into what could commonly be called 'guerrilla' warfare.

What this meant was that 'guerrilla' forces who engaged the government or the state in a civil war, often did not have access to a standing army in the same way the state did. Consequently, the need to create a military force from a sometimes unwilling civilian population saw the abduction of younger civilians, i.e. male and female children. As Alcinda Honwana argues in her book Child Soldiers in Africa (Philadelphia, 2006), children are in a process of formation and development, and therefore they can be more readily and more easily guided by an authority figure into becoming good soldiers. That such 'recruitment' occurs in nations or regions undergoing often violent and chaotic conflict or turmoil essentially prevents the social structure (families, village groups, etc) from being able to resist. Honwana, who focused her study predominantly on Mozambique and Angola and the use of child soldiers during the 1970s, 1980s, and 1990s, contends that child soldiering was not a result simply of this ability to ideologically manipulate children, nor that a shortage of manpower can fully explain their use. Instead, she concludes that the use of child soldiers was an intrinsic part of warfare strategies for some African military leaders, whether guerrillas fighting against the state, or political groups fighting against other political factions.

So to answer your question, child soldiers in the modern sense can really only be seen as having been utilised since the mid-1980s, not because children were not used in conflict before that, but because the role they played and the international humanitarian laws and social recognition of childhood have changed. Children are now more protected, especially in situations where the usual social systems like family cannot protect them.

Have a look at Alcinda Honwana's book that I mentioned above, as it really provides a good introduction (albeit with a limited focus). Any questions please fire away!

So I'll preface by saying that Africa is a large continent with lots of different ethnic groups. The age of adulthood in many areas is historically different to the 18/21 that we seem to use nowadays. The precedent for Child Soldiers in the much of Africa can be traced back to pre-colonial times. I'll give a couple of examples of societies where children by our standards may be expected to fight.

Nguni

One group that has historically used children for soldiers for a long time is the Nguni peoples. The Nguni is a term for the peoples in Southern and Central Africa that have historically had a very militarized society. After or during the Mfecane in the early 19th century these peoples started to migrate north and form more centralized states. The most famous group or state is the Zulu, but other similar groups such as the Ndebele and the Ngoni exist too. In many Nguni societies age-set regiments are used which mean that youths from the age of around 14-15 are grouped in a statewide regiment in which they all train to be warriors. The only way to progress in these societies is usually to "bloody the spear" or kill another warrior in hand to hand combat. The more kills one gets the more prestige. Children of around 14-15 may have seen combat if they are particularly gifted or it was necessary for them to fight. Most people fighting would be considered men in the society rather than children.

Arab-Swahili Child Soldiers

Despite existing and roughly the same time the Arab-Swahili Child Soldiers were very different to child soldiers in Nguni cultures. There is no warrior mentality or requirement to kill in hand to hand combat. Arab-Swahili merchants would travel through eastern central Africa, plundering villages as they went. Due to superior firepower and shrewd diplomacy among locals there was little resistance. On the way they would take many slaves and often recruit child soldiers or depends in order to fill their ranks. We know from his autobiography that Tippu Tip used child soldiers. Some of them would actually have seen combat, many would have been porters or involved in the logistical side of operations. The child soldiers were generally loyal to the merchant they were serving and had a decent chance of upward mobility. We have examples of child soldiers growing up and leading merchant raiding further into the continent. This is perhaps most like the Child Soldiers that we see today, however we must ground them in the time the existed. We would likely see children acting as porters and auxiliary roles in European and eastern armies too.

Hopefully this shows some of the precedents for Child Soldiers in Africa. There are other examples across the continent, some that will no doubt be vastly different to the two I have outlined. The use of child soldiers is not a uniquely African thing during this time, as I understand Children served in auxiliary position in European armies too.