Musannaf Ibn Abi Shayba 36:232

'Abdullah ibn Idris narrated to us, from 'Ubaydullah ibn 'Umar, from Nafi', from Ibn 'Umar, who said: I was presented to the Messenger of Allah (may Allah bless him and grant him peace) on the day of Uhud, and I was fourteen years old, so he deemed me too young. I was presented to him on the day of the Trench (al-Khandaq), and I was fifteen years old, so he permitted me. Nafi' said: I narrated this to 'Umar ibn 'Abd al-'Aziz, and he said: "This is the boundary between the young and the adult." So he wrote to his governors to enroll those who were fifteen years old among the combatants, and those who were fourteen years old among the dependents (dhurriyyah).