Narrated Ibn 'Umar (RA):

I was brought before the Prophet (ﷺ) on the Day of Uhud when I Was fourteen years old, but he did not give me permission (to fight). I was afterwards brought to him on the Day of al-Khandaq (the Battle of Trench) when I was fifteen years of age, and he gave me permission (to fight). [Agreed upon].

وَعَنِ اِبْنِ عُمَرَ ‏-رَضِيَ اَللَّهُ عَنْهُمَا‏- قَالَ: { عُرِضْتُ عَلَى اَلنَّبِيِّ ‏- صلى الله عليه وسلم ‏-يَوْمَ أُحُدٍ, وَأَنَا اِبْنُ أَرْبَعَ عَشْرَةَ سَنَةً, فَلَمْ يُجِزْنِي, وَعُرِضْتُ عَلَيْهِ يَوْمَ اَلْخَنْدَقِ, وَأَنَا اِبْنُ خَمْسَ عَشْرَةَ سَنَةً, فَأَجَازَنِي } مُتَّفَقٌ عَلَيْهِ [1]‏ .‏