Musannaf Ibn Abi Shayba 35:158

Ibn Idris narrated to us, from Ubayd Allah ibn Umar, from Nafi, from Ibn Umar, who said: I was presented to the Prophet (ﷺ) on the day of Uhud while I was fourteen years old, and he deemed me too young. I was presented to him on the day of the Trench (al-Khandaq) while I was fifteen years old, and he permitted me. Nafi said: I narrated this to Umar ibn Abd al-Aziz. He said: "This is the boundary between the young and the adult." He then wrote to his governors to enlist those who are fifteen years old among the combatants, and those who are fourteen years old among the dependents (dhurriyyah). And it was mentioned that Abu Hanifah said: There is nothing [required] of a young girl until she reaches eighteen or seventeen.