Children in the military
History: History is filled with children who have been trained and used for fighting, assigned to support roles such as porters or messengers, used as sex slaves, or recruited for tactical advantage as human shields or for political advantage in propaganda. In 1813 and 1814, for example, Napoleon conscripted many young French teenagers for his armies. Thousands of children participated on all sides of the First and Second World Wars. Children continued to be used throughout the 20th and early 21st century on every continent, with concentrations in parts of Africa, Latin America, and the Middle East. Only since the turn of the millennium have international efforts begun to limit and reduce the military use of children.

Current situation: 

State armed forces: The adoption in 2000 of the Optional Protocol on the Involvement of Children in Armed Conflict (OPAC) committed states who ratified it to "take all feasible measures" to ensure that no child takes a direct part in hostilities and to cease recruitment below the age of 16. As most states have now opted into OPAC, the global trend has been towards reserving military recruitment to adulthood, known as the Straight-18 standard.
Nonetheless, as of 2018, children aged under 18 were still being recruited and trained for military purposes in 46 countries, which is approximately one quarter of all countries. Most of these states recruit from age 17, fewer than 20 recruit from age 16, and an unknown, smaller number, recruit younger children.
As of 2022, the United Nations (UN) verified that nine state armed forces were using children in hostilities: Central African Republic, Democratic Republic of the Congo, Mali, Somalia, and South Sudan in Africa; Palestine, Syria and Yemen in Western Asia; Afghanistan in Central Asia; and Myanmar in South East Asia.
The United Nations (UN) Committee on the Rights of the Child and others have called for an end to the recruitment of children by state armed forces, arguing that military training, the military environment, and a binding contract of service are not compatible with children's rights and jeopardize healthy development.

Non-state armed groups: These include non-state armed paramilitary organisations such as militias, insurgents, terrorist organizations, guerrilla movements, armed liberation movements, and other types of quasi-military organisation.
As of 2022, the UN identified 12 countries where children were widely used by such groups: Colombia in South America; Central African Republic, Democratic Republic of the Congo, Mali, Somalia, South Sudan, and Sudan in Africa; Lebanon and Palestine in the Middle East; Syria and Yemen in Western Asia; Afghanistan in Central Asia; and Myanmar in South East Asia.
Not all armed groups use children and approximately 60 have entered agreements to reduce or end the practice since 1999. For example, by 2017, the Moro Islamic Liberation Front (MILF) in the Philippines had released nearly 2,000 children from its ranks, and in 2016, the FARC-EP guerrilla movement in Colombia agreed to stop recruiting children. Other countries have seen the reverse trend, particularly Afghanistan and Syria, where Islamist militants and groups opposing them have intensified their recruitment, training, and use of children.

Global estimates: In 2003, one estimate calculated that child soldiers participated in about three-quarters of ongoing conflicts. In the same year, the UN Office for the Coordination of Humanitarian Affairs (UNOCHA) estimated that most of these children were aged over 15, although some were younger.
Due to the widespread military use of children in areas where armed conflict and insecurity prevent access by UN officials and other observers, it is difficult to estimate how many children are affected. 

In 2003 UNICEF estimated that some 300,000 children are involved in more than 30 conflicts worldwide.
In 2017, Child Soldiers International estimated that several tens of thousands of children, possibly more than 100,000, were in state and non-state military organisations around the world, and in 2018 the organisation reported that children were being used to participate in at least 18 armed conflicts.
In 2023 the UN Secretary General report presented 7,622 verified cases of children being recruited and used in armed conflicts in 23 countries. More than 12,460 children formerly associated with armed forces or groups received protection or reintegration support during 2022.
It is estimated that girl soldiers make between 10% and 30%, 6 and 50%, or over 40% of the child soldier population. Of the verified cases presented in the 2023 UN Secretary General report, girls make 12.3% of all child soldiers recruited or used by armed groups.

Driving factors: Despite children's physical and psychological underdevelopment relative to adults, there are many reasons why state and non-state military organisations seek them out, and why children themselves are often drawn to join of their own volition.

Psychological factors: Relative to adults, the neurological underdevelopment of children, including adolescent children, renders them more susceptible to recruitment and also more likely to make consequential decisions without due regard to the risks.
With these susceptibilities in mind, military marketing to adolescents has been criticised in Germany, the UK, and the US for glamorizing military life while omitting the risks and the loss of fundamental rights.
Research in the same three countries finds that recruiters disproportionately target children from poorer backgrounds. In the UK, for example, the army finds it easier to attract child recruits from age 16 than adults from age 18, particularly those from poorer backgrounds.
Once recruited, children are easier than adults to indoctrinate and control, and are more motivated than adults to fight for non-monetary incentives such as religion, honour, prestige, revenge, and duty.

Social factors: In many countries growing populations of young people relative to older generations have made children a cheap and accessible resource for military organisations. In a 2004 study of children in military organisations around the world, Rachel Brett and Irma Specht pointed to a complex of factors that incentivise children to join military organisations, particularly:

Background poverty including a lack of civilian education or employment opportunities.
The cultural normalization of war.
Seeking new friends.
Revenge (for example, after seeing friends and relatives killed).
Expectations that a "warrior" role provides a rite of passage to maturity.
The following testimony from a child recruited by the Cambodian armed forces in the 1990s is typical of many children's motivations for joining up:

I joined because my parents lacked food and I had no school ... I was worried about mines but what can we do—it's an order [to go to the front line]. Once somebody stepped on a mine in front of me—he was wounded and died ... I was with the radio at the time, about 60 metres away. I was sitting in my hammock and saw him die ... I see young children in every unit ... I'm sure I'll be a soldier for at least a couple of more years. If I stop being a soldier, I won't have a job to do because I don't have any skills. I don't know what I'll do ...

Military factors: Some leaders of armed groups have claimed that children, despite their underdevelopment, bring their own qualities as combatants to a fighting unit, often being remarkably fearless, agile and hardy.
The global proliferation of light automatic weapons, which children can easily handle, has also made the use of children as direct combatants more viable.

Impact on children: 

Armed conflict: Child soldiers who survive armed conflict face a markedly elevated risk of debilitating psychiatric illness, poor literacy and numeracy, and behavioural problems. Research in Palestine and Uganda, for example, has found that more than half of former child soldiers showed symptoms of post-traumatic stress disorder and nearly nine in ten in Uganda screened positive for depressed mood. Researchers in Palestine also found that children exposed to high levels of violence in armed conflict were substantially more likely than other children to exhibit aggression and anti-social behaviour. The combined impact of these effects typically includes a high risk of poverty and lasting unemployment in adulthood.

Detention: Further harm is caused when armed forces and groups detain child recruits. Children are often detained without sufficient food, medical care, or under other inhumane conditions, and some experience physical and sexual torture. Some are captured with their families, or detained due to one of their family members' activity. Lawyers and relatives are frequently banned from any court hearing.

Military training: While the use of children in armed conflict has attracted most attention, other research has found that military settings present several serious risks before child recruits are deployed to war zones, particularly during training.
Research from several countries finds that military enlistment, even before recruits are sent to war, is accompanied by a higher risk of attempted suicide in the US, higher risk of mental disorders in the US and the UK, higher risk of alcohol misuse and higher risk of violent behaviour, relative to recruits' pre-military experience.
Military academics in the US have characterized military training as "intense indoctrination" in conditions of sustained stress, the primary purpose of which is to establish the unconditional and immediate obedience of recruits. The research literature has found that adolescents are more vulnerable than adults to a high-stress environment, particularly those from a background of childhood adversity. It finds in particular that the prolonged stressors of military training are likely to aggravate pre-existing mental health problems and hamper healthy neurological development.
Military settings are characterized by elevated rates of bullying, particularly by instructors. In the UK between 2014 and 2020, for example, the army recorded 62 formal complaints of violence committed by staff against recruits at the military training centre for 16- and 17-year-old trainee soldiers, the Army Foundation College.