It was narrated from Ibn 'Umar that he presented himself to the Messenger of Allah on the Day of Uhud when he was fourteen years old, but he did not permit him (to join the army). He presented himself on the Day of Al-Khandaq when he was fifteen years old, and he permitted him (to join the army).  

أَنَّ رَسُولَ اللَّهِ ﷺ عَرَضَهُ يَوْمَ أُحُدٍ وَهُوَ ابْنُ أَرْبَعَ عَشْرَةَ سَنَةً فَلَمْ يُجِزْهُ وَعَرَضَهُ يَوْمَ الْخَنْدَقِ وَهُوَ ابْنُ خَمْسَ عَشْرَةَ سَنَةً فَأَجَازَهُ  

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  

[Machine] "I was presented to the Messenger of Allah ﷺ when I was fourteen years old, but he did not give me permission and did not take notice of me. Then, I was presented to him when I was fifteen years old and he granted me permission."  

«عُرِضْتُ عَلَى رَسُولِ اللَّهِ ﷺ وَأَنَا ابْنُ أَرْبَعَ عَشْرَةَ سَنَةً فَلَمْ يُجِزْنِي وَلَمْ يَرَنِي بَلَغْتُ ثُمَّ عُرِضْتُ عَلَيْهِ وَأَنَا ابْنُ خَمْسَ عَشْرَةَ سَنَةً فَأَجَازَنِي»  

[Machine] "My father brought me to the Messenger of Allah ﷺ on the day of Uhud, and I was fourteen years old. The Prophet ﷺ did not allow me. Then my father brought me on the day of the trench, and I was fifteen years old. So the Messenger of Allah ﷺ assigned me." Naafi' said, "I narrated this to Umar ibn Abd al-Aziz, and he ordered that it should not be assigned except for someone who is fifteen years old."  

«جَاءَ بِي أَبِي يَوْمَ أُحُدٍ إِلَى رَسُولِ اللهِ ﷺ وَأَنَا ابْنُ أَرْبَعَ عَشْرَةَ فَلَمْ يُجِزْنِي النَّبِيُّ ﷺ ثُمَّ جَاءَ بِي يَوْمَ الْخَنْدَقِ وَأَنَا ابْنُ خَمْسَ عَشْرَةَ فَفَرَضَ لِي رَسُولُ اللهِ ﷺ» قَالَ نَافِعٌ «فَحَدَّثْتُ بِهِ عُمَرَ بْنَ عَبْدِ الْعَزِيزِ فَأَمَرَ بِأَنْ لَا يُفْرَضَ إِلَّا لِابْنِ خَمْسَ عَشْرَةَ»  

"عن ابن عمر قال: عُرِضْتُّ عَلَى النَّبِىِّ ﷺ يَوْمَ أُحُدٍ وأَنَا ابْنُ أَرْبَعَ عَشْرَةَ سَنَةً، فَلَمْ يُجِزْني وَلَمْ يَرَنِى بَلَغْتُ، وعُرِضْتُّ عَلَيْهِ يَوْمَ الْخَنْدَقِ وَأَنَا ابْنُ خَمْسَ عَشْرةَ فَأَجَازَنِى".  

"عُرِضْتُ عَلَى النَّبِىِّ ﷺ يَوْمَ أُحُدٍ وَأَنَا ابْنُ أَرْبَعَ عَشْرَةَ سَنَةً فَاسْتَصْغَرَنِى، وَعُرِضْتُ عَلَيْهِ يَوْمَ الْخَنْدَقِ وَأَنَا ابْنُ خَمْسَ عَشْرَةَ فَأَجَازَنِى".  

"عَنِ ابْنِ عُمَرَ قَالَ: عُرِضْتُ عَلَى النَّبِىِّ ﷺ يَوْمَ بَدْرٍ وَأَنَا ابْنُ ثَلاَثَ عَشْرَةَ فَرَدَّنِى، ثُمَّ عُرِضْتُ عَلَيْهِ يَوْمَ أُحُد وَأَنَا ابْنُ أَرْبَعَ عَشْرَةَ سَنَةً فَرَدَّنِى، ثُمَّ عُرِضْتُ عَلَيْهِ يَوْمَ الْخَنْدَقِ وَأَنَا ابْنُ خَمْسَ عَشْرَةَ سَنَةً فَأَجَازَنِى ".