Sunan an-Nasa'i, Volume 4, Book 27, Hadith 3461 (DarusSalam) ⓘ

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).

أَخْبَرَنَا عُبَيْدُ اللَّهِ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا يَحْيَى، عَنْ عُبَيْدِ اللَّهِ، قَالَ أَخْبَرَنِي نَافِعٌ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم عَرَضَهُ يَوْمَ أُحُدٍ وَهُوَ ابْنُ أَرْبَعَ عَشْرَةَ سَنَةً فَلَمْ يُجِزْهُ وَعَرَضَهُ يَوْمَ الْخَنْدَقِ وَهُوَ ابْنُ خَمْسَ عَشْرَةَ سَنَةً فَأَجَازَهُ ‏.‏