Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: On the day of Uhud I was presented before the Messenger of Allah (peace and blessings be upon him), and at that time I was fourteen years old, and he (peace and blessings be upon him) considered me too young. Then the following year, at the time of Khandaq, I was presented, and at that time I was fifteen years old, and he (peace and blessings be upon him) let me go to fight. In a hadith of Sahih Muslim the words are: "He (peace and blessings be upon him) considered me too young and sent me back to the boys."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحجر / 11300