ENOfficial
Ibn ‘Umar said: 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.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا أَحْمَدُ بْنُ حَنْبَلٍ، حَدَّثَنَا يَحْيَى، عَنْ عُبَيْدِ اللَّهِ، قَالَ أَخْبَرَنِي نَافِعٌ، عَنِ ابْنِ عُمَرَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم عُرِضَهُ يَوْمَ أُحُدٍ وَهُوَ ابْنُ أَرْبَعَ عَشْرَةَ سَنَةً فَلَمْ يُجِزْهُ وَعُرِضَهُ يَوْمَ الْخَنْدَقِ وَهُوَ ابْنُ خَمْسَ عَشْرَةَ سَنَةً فَأَجَازَهُ .
sunnah.com