Nafi' narrates from Sayyiduna Ibn Umar (may Allah be pleased with them both): I was presented before the Prophet (peace and blessings be upon him) on the day of Uhud, when I was fourteen years old, and he (peace and blessings be upon him) considered me too young. On the day of the Trench (Khandaq) I was fifteen years old, and then he (peace and blessings be upon him) gave me permission.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5088