From Sahih Bukhari (Hadith 4097):
Narrated Ibn `Umar:That the Prophet (ﷺ) inspected him on the day of Uhud while he was fourteen years old, and the Prophet (ﷺ) did not allow him to take part in the battle. He was inspected again by the Prophet (ﷺ) on the day of Al- Khandaq (i.e. battle of the Trench) while he was fifteen years old, and the Prophet (ﷺ) allowed him to take Part in the battle
Reference: Bukhari 4097