Ubaydullah (may Allah have mercy on him) narrated the like of the aforementioned hadith, except that it has this addition: "And on the day of Khandaq he looked me over when I was fifteen years old, and he (peace and blessings be upon him) let me go." Nafi' (may Allah have mercy on him) said: I came to Umar bin Abd al-Aziz (may Allah have mercy on him), then he narrated the hadith, and he also said: "And whoever is younger than that, put him with his family as a dependent."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحجر / 11298