Sayyidina Ibn Umar (RA) narrated: I was presented before Allah’s Messenger (SAW) for the army when! was fourteen years old. But, he did not enlist me. Then, the next year I was presented to him for the army being fifteen years and he took me.
Nafi’ said: I narrated this hadith to Umar ibn Abdul Aziz and he said, “The limit between a minor and a major (is this).” Then he wrote (to his officers) that those who attain the age of fifteen may be given share in spoils of war.
[Bukhari 2664, Muslim 1868]