Sayyiduna Ibn Umar (may Allah be pleased with them both) said: Rafi' bin Khadij and I were presented before the Prophet (peace and blessings be upon him) on the day of the Trench (for battle), and at that time each of us was fifteen years old, so he accepted us both.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17808