It was narrated that Ibn`Umar said: “I was presented to the Messenger of Allah (ﷺ) on the day of Uhud, when I was fourteen years old, but he did not permit me (to fight). I was presented to him on the Day of Khandaq when I was fifteen years old, and he permitted me (to fight).' ”
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَلِيُّ بْنُ مُحَمَّدٍ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ نُمَيْرٍ، وَأَبُو مُعَاوِيَةَ وَأَبُو أُسَامَةَ قَالُوا حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ عُمَرَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ عُرِضْتُ عَلَى رَسُولِ اللَّهِ صلى الله عليه وسلم يَوْمَ أُحُدٍ وَأَنَا ابْنُ أَرْبَعَ عَشْرَةَ سَنَةً فَلَمْ يُجِزْنِي وَعُرِضْتُ عَلَيْهِ يَوْمَ الْخَنْدَقِ وَأَنَا ابْنُ خَمْسَ عَشْرَةَ سَنَةً فَأَجَازَنِي . قَالَ نَافِعٌ فَحَدَّثْتُ بِهِ عُمَرَ بْنَ عَبْدِ الْعَزِيزِ فِي خِلاَفَتِهِ فَقَالَ هَذَا فَصْلُ مَا بَيْنَ الصَّغِيرِ وَالْكَبِيرِ .
sunnah.com