Mishkat al-Masabih 3376
Ibn ‘Umar said:
I was brought before God’s Messenger in the year of Uhud when I was fourteen and he sent me back. Then I was brought before him in the year of the Trench when I was fifteen and he gave me permission to fight.* ‘Umar b. ‘Abd al-‘Aziz said this represents the difference between fighting men and children. 
* Ajazani . It may be translated as above or it may mean “he gave me a fighting man's permission”.
(Bukhari and Muslim.)
عَنِ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَا قَالَ: عُرِضْتُ عَلَى رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَامَ أُحُدٍ وَأَنَا ابْنُ أَرْبَعَ عَشْرَةَ سَنَةً فَرَدَّنِي ثُمَّ عُرِضْتُ عَلَيْهِ عَامَ الْخَنْدَقِ وَأَنَا ابْنُ خَمْسَ عَشْرَةَ سَنَةً فَأَجَازَنِي فَقَالَ عُمَرُ بْنُ عَبْدِ الْعَزِيزِ: هَذَا فَرْقُ مَا بَين الْمُقَاتلَة والذرية