I was presented to the Prophet ﷺ on the day of Uhud when I was fourteen years old, and he did not permit me to take part. Then I was presented to him on the day of the Trench when I was fifteen years old, and he permitted me.
عرضت على النبي ﷺ يوم أحد وأنا ابن أربع عشرة سنة فلم يجزني، وعرضت عليه يوم الخندق وأنا ابن خمس عشرة سنة فأجازني.
Bukhari narrated it in the Book of Testimonies (2664), and Muslim in the Book of Leadership (1868), both from the hadith of Ubaydullah, who said, Nafi told me, who said, Ibn Umar told me, and he related it.Nafi said, I came to Umar bin Abdul-Aziz while he was caliph, and I told him this hadith. He said, "This is indeed the boundary between the young and the old." And he wrote to his governors instructing them to grant stipends to whoever reached the age of fifteen.