That Ibn 'Umar was presented before the Prophet ﷺ on the day of Uhud, when he was fourteen years old, but he did not allow him. He was again presented to him on the day of Khandaq (the battle of Trench) when he was fifteen years old, he allowed him. (Using translation from Abū Dāʾūd 2957)  

أَنَّ رَسُولَ اللهِ ﷺ «عَرَضَهُ يَوْمَ أُحُدٍ وَهُوَ ابْنُ أَرْبَعَ عَشْرَةَ فَلَمْ يُجِزْهُ وَعَرَضَهُ يَوْمَ الْخَنْدَقِ وَهُوَ ابْنُ خَمْسَ عَشْرَةَ فَأَجَازَهُ»