Sayyiduna Ibn Umar (may Allah be pleased with them both) said: I was presented before the Messenger of Allah (peace and blessings be upon him) on the day of Uhud when I was fourteen years old, and at that age he (peace and blessings be upon him) considered me too young. And on the day of the Trench (Khandaq) I was presented before the Messenger of Allah (peace and blessings be upon him), when I was fifteen years old, and at that age he (peace and blessings be upon him) accepted me.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السرقة / 17207