Sahih Bukhari | Volume 5 | Hadith Number 423

Narrated by Ibn Umar
That the Prophet inspected him on the day of Uhud while he was fourteen years old, and the Prophet did not allow him to take part in the battle. He was inspected again by the Prophet on the day of Al-Khandaq (i.e. battle of the Trench) while he was fifteen years old, and the Prophet allowed him to take Part in the battle.