UN Convention on the Rights of a Child:  Despite the document general definition of a child being a person under eighteen years of age, Article 38 (3) permits the recruitment of adolescents at the age fifteen. 

"States Parties shall refrain from recruiting any person who has not attained the age of fifteen years into their armed forces. In recruiting among those persons who have attained the age of fifteen years but who have not attained the age of eighteen years, States Parties shall endeavor to give priority to those who are oldest."