Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: On the day of Badr I was presented before the Prophet (peace and blessings be upon him) when I was thirteen years old, and he (peace and blessings be upon him) did not include me among the fighters. Then I was presented on the day of Uhud, when I was fourteen years old, and he (peace and blessings be upon him) did not include me among the fighters. Then I was presented on the day of Khandaq, when I was fifteen years old, and he (peace and blessings be upon him) included me among the fighters.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحجر / 11302