Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) looked me over on the day of the Battle of Uhud, when I was fourteen years old. He (peace and blessings be upon him) considered me too young and sent me back among the boys. When the day of the Battle of Khandaq came, he (peace and blessings be upon him) looked me over, and by then I had become fifteen years old. He (peace and blessings be upon him) permitted me to take part in jihad. Ubaydullah said: And Umar bin Abd al-Aziz (may Allah have mercy on him) wrote that those of fifteen years should be included in the obligation (of jihad). Another hadith has these words: The Prophet (peace and blessings be upon him) accepted Sayyiduna Ibn Umar and Sayyiduna Rafi' bin Khadij (may Allah be pleased with them both) on the day of the Battle of Khandaq, and at that time they were fifteen years old.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحجر / 11301