Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: On the day of Uhud, the Messenger of Allah (peace and blessings be upon him) looked me over for fighting when I was fourteen years old, and he (peace and blessings be upon him) did not permit me. Then when the time of Khandaq came, I was fifteen years old, and he (peace and blessings be upon him) permitted me. Later I came to Umar bin Abd al-Aziz (may Allah have mercy on him), who was the caliph in those days, and narrated this hadith to him. He said: This is the dividing line between the young and the grown-up. And he wrote to his governors: Make (the allotment) due to those who are fifteen, and attach those who are younger than that to their families as dependents.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحجر / 11297