Children in the military
Throughout the Syrian Civil War multiple media outlets including Human Rights Watch have confirmed that the YPG, an organization linked to the PKK, has been recruiting and deploying child soldiers. Despite a claim by the group that it would stop using children, which has been violation of international law, the group has continued the recruitment and use of children.
In 2018, the annual UN report on children in armed conflict found 224 cases of child recruitment by the People's Defense Units and its women's unit in 2017, an almost fivefold increase from the 2016. Seventy-two of the children, nearly one-third, were girls. The group was also reported to had abducted children to enlist them.

Yemen: U.N. Special Representative for Children and Armed Conflict Radhika Coomaraswamy stated in January 2010 that "large numbers" of teenage boys are being recruited in Yemeni tribal fighting. NGO activist Abdul-Rahman al-Marwani has estimated that as many as 500–600 children are either killed or wounded through tribal combat every year in Yemen.
Saudi Arabia hired child soldiers from Sudan (especially from Darfur), and Yemen to fight against Houthis during the Yemeni Civil War (2015–present).
British SAS special forces are allegedly involved in training child soldiers in Yemen. Reportedly at least 40% of soldiers fighting for the Saudi-led coalition are children.
Saudi Arabia is also hiring Yemeni child soldiers to guard Saudi border against Houthis.
In June 2019, Mike Pompeo, the US Secretary of State, blocked the inclusion of Saudi Arabia on the US list of countries that recruit child soldiers, dismissing his experts' findings that a Saudi-led coalition has been using children in Yemen's civil war.
Male Children can volunteer for military service at 17 in Saudi Arabia. (CIA World Factbook - 2024)

Europe: According to Child Soldiers International the trend in Europe has been towards recruiting only adults from age 18; most states only allow adult recruitment, and as of 2016 no armed groups were known to be using children. As of 2018 one country, the United Kingdom, was enlisting children from age 16, and five were enlisting from age 17 (Austria, Cyprus, France, Germany, and Netherlands). Of these, the UK recruits children in the greatest numbers; in 2016, approximately a quarter of new recruits to the British army were aged under 18.
All European states have ratified the Optional Protocol on the Involvement of Children in Armed Conflict, and so child recruits are not typically used in hostilities until they reach adulthood. Children were used as combatants in the First Chechen War during the 1990s.

Austria: Austria invites male children to begin their adult compulsory military service one year early, at age 17, with the consent of their parents.

Belarus: AT 17 children can join the military higher education instatutes where they are classified as military personnel. (CIA Factbook 2024)

Cyprus: Cyprus invites children to begin their adult compulsory military service two years early, at age 16, with the consent of their parents.

France: France enlists military personnel from age 17 and 6 months, and students for military technical school from age 16; 3% of its armed forces' intake is aged under 18.

Germany: Germany enlists military personnel from age 17; in 2015 6% of its armed forces' intake was aged under 18.

Italy: Children can join the Italian military at age 17. (CIA Factbook 2024)

Netherlands: The Netherlands enlists military personnel from age 17; in 2014 5% of its armed forces' intake was aged under 18.

Norway: Male children can volunteer to join at 17. (CIA Factbook 2024)

Ukraine: During the armed conflict in Eastern Ukraine in 2014 Justice for Peace at Donbas documented 41 verified individual cases of child recruitment into armed formations. Of those 37 concerned the participation of children in armed formations on territory occupied by Russia and 4 on territory controlled by Ukraine. There were 31 further reports of child recruitment which could not be verified. Of the 37 verified cases on territory controlled by Russia, 33 were boys and 4 were girls; 57% were aged 16–17, 35% were under 15, and age could not be determined in 8% of cases.
In December 2022, Andrii Yusov, a representative of Ukraine's Defence Intelligence, stated 17-year-olds have been drafted by the Russian military in occupied territories.

United Kingdom: The British armed forces enlist personnel from age 16 and accept applications from children aged 15 years, 7 months. Parental consent is required prior to enlistment.
As of 2022, 23% of enlistees to the British armed forces were aged under 18. Most child recruits were enlisted for the army, where 30% of the intake in the year 2021-2022 was aged under 18; more new soldiers were 16 than any other age.
Army recruits aged between 16 and 17.5 train initially at the Army Foundation College, a military training centre dedicated to the age group. As of 2021, the centre is graded 'outstanding' for welfare by the education inspectorate Ofsted, although multiple allegations of abuse at the centre, some proven, are on the army's record. In 2021, nine investigations were opened into sexual offences against 22 girls at the centre; in one investigation, three of the suspected perpetrators were members of staff. The revelations were the subject of a Vice News report in July 2022.
As per the OPAC, the UK does not normally send child recruits to participate in hostilities, although it does not rule out doing so.
The UK inadvertently deployed 22 personnel aged under 18 to Iraq and Afghanistan between 2003 and 2010. The committee on the Rights of the Child has urged the UK to alter its policy so as to ensure that children cannot take part in hostilities under any circumstances.
In negotiations on the OPAC during the 1990s the UK joined the US in opposing a global minimum enlistment age of 18. Children's rights bodies have criticised the UK's continuing reliance on children to staff its armed forces.

Oceania: 

Australia: The Australian Defence Force allows personnel to enlist with parental consent from the age of 16. Personnel under the age of 18 cannot be deployed overseas or used in direct combat except in extreme circumstances where it is not possible to evacuate them.

New Zealand: As of 2018, the minimum age for joining the New Zealand Defence Force was 17.

Movement to end military use of children: The military use of children has been common throughout history; only in recent decades has the practice met with informed criticism and concerted efforts to end it. Progress has been slow, partly because many armed forces have relied on children to fill their ranks, and partly because the behaviour of non-state armed groups is difficult to influence.

Recent history: 

1970s–1980s: International efforts to limit children's participation in armed conflict began with the Additional Protocols to the 1949 Geneva Conventions, adopted in 1977 (Art. 77.2). The new Protocols prohibited the military recruitment of children aged under 15, but continued to allow state armed forces and non-state armed groups to recruit children from age 15 and use them in warfare.
Efforts were renewed during negotiations on the Convention on the Rights of the Child (CRC), when Non-governmental organisations (NGOs) campaigned for the new treaty to outlaw child recruitment entirely. Some states, whose armed forces relied on recruiting below the age of 18, resisted this, so the final treaty text of 1989 only reflected the existing legal standard: the prohibition of the direct participation of children aged under 15 in hostilities.

1990s: In the 1990s NGOs established the Coalition to Stop the Use of Child Soldiers (now Child Soldiers International) to work with sympathetic governments on a campaign for a new treaty to correct the deficiencies they saw in the CRC. After a global campaign lasting six years, the treaty was adopted in 2000 as the Optional Protocol on the Involvement of Children in Armed Conflict (OPAC). The treaty prohibits child conscription, ensures that military recruits are no younger than 16, and forbids the use of child recruits in hostilities. The treaty also forbids non-state armed groups from recruiting anyone under the age of 18 for any purpose. Although most states negotiating OPAC supported a ban on recruiting children, some states, led by the US in alliance with the UK, objected to this. As such, the treaty does not ban the recruitment of children aged 16 or 17, although it allows states to bind themselves to a higher standard in law.

2000s–present: After the adoption of the Optional Protocol on the Involvement of Children in Armed Conflict, a campaign for global ratification made swift progress. As of 2018 OPAC had been ratified by 167 states. The campaign also successfully encouraged many states not to recruit children at all. In 2001 83 states only allowed adult enlistment. By 2016 this had increased to 126, which is 71 percent of countries with armed forces. Approximately 60 non-state armed groups have also entered agreements to stop or scale back their use of children, often brokered by the UN or the NGO Geneva Call.