I was among the captives of Banu Qurayzah. They (the Companions) examined us, and those who had begun to grow hair (pubes) were killed, and those who had not were not killed. I was among those who had not grown hair.  

كُنْتُ مِنْ سَبْىِ بَنِي قُرَيْظَةَ فَكَانُوا يَنْظُرُونَ فَمَنْ أَنْبَتَ الشَّعْرَ قُتِلَ وَمَنْ لَمْ يُنْبِتْ لَمْ يُقْتَلْ فَكُنْتُ فِيمَنْ لَمْ يُنْبِتْ  

They uncovered my private parts, and when they found that the hair had not begun to grow they put me among the captives.  

فَكَشَفُوا عَانَتِي فَوَجَدُوهَا لَمْ تَنْبُتْ فَجَعَلُونِي فِي السَّبْىِ  

He was presented before the prophet ﷺ on the day of Uhd when he was fourteen years old, but he did not allow him (to participate in the battle). He was again presented before him on the day of Khandaq when he was fifteen years old, Then he allowed him.  

أَنَّ النَّبِيَّ ﷺ عُرِضَهُ يَوْمَ أُحُدٍ وَهُوَ ابْنُ أَرْبَعَ عَشْرَةَ سَنَةً فَلَمْ يُجِزْهُ وَعُرِضَهُ يَوْمَ الْخَنْدَقِ وَهُوَ ابْنُ خَمْسَ عَشْرَةَ سَنَةً فَأَجَازَهُ  

When I mentioned this tradition to ‘Umar.b.’Abd al-Aziz he said : This prescribed punishment is between the minor and the major.  

إِنَّ هَذَا الْحَدُّ بَيْنَ الصَّغِيرِ وَالْكَبِيرِ