Jami` at-Tirmidhi, Volume 3, Book 21, Hadith 1711 (DarusSalam) ⓘ

Narrated Nafi: That Ibn 'Umar said: "I was reviewed before army, and I was fourteen years old, but he did not accept me. Then I was reviewed before him later in army while I was fifteen years old, and he accepted me." Nafi said: "I narrated this Hadith to 'Umar bin 'Abdul-'Aziz and he said: 'This is the limit that distinguishes between youth and manhood.' The he wrote to give salaries to whoever reached fifteen years of age.

حَدَّثَنَا مُحَمَّدُ بْنُ الْوَزِيرِ الْوَاسِطِيُّ، حَدَّثَنَا إِسْحَاقُ بْنُ يُوسُفَ الأَزْرَقُ، عَنْ سُفْيَانَ، عَنْ عُبَيْدِ اللَّهِ بْنِ عُمَرَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ عُرِضْتُ عَلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فِي جَيْشٍ وَأَنَا ابْنُ أَرْبَعَ عَشَرَةَ فَلَمْ يَقْبَلْنِي ثُمَّ عُرِضْتُ عَلَيْهِ مِنْ قَابِلٍ فِي جَيْشٍ وَأَنَا ابْنُ خَمْسَ عَشَرَةَ فَقَبِلَنِي ‏.‏ قَالَ نَافِعٌ فَحَدَّثْتُ بِهَذَا الْحَدِيثِ عُمَرَ بْنَ عَبْدِ الْعَزِيزِ فَقَالَ هَذَا حَدُّ مَا بَيْنَ الصَّغِيرِ وَالْكَبِيرِ ‏.‏ ثُمَّ كَتَبَ أَنْ يُفْرَضَ لِمَنْ بَلَغَ الْخَمْسَ عَشَرَةَ ‏.‏

Another chain with similar, but he (Nafi') said: "Umar [bin 'Abdul-'Aziz] said: 'This is the limit that distinguished between children and soldiers.'" And he did not mention him writing about the salary [Abu 'Eisa said:] The Hadith of Ishaq bin Yusuf is a Hasan Sahih Gharib Hadith as narration of Sufyan Ath-Thawri

حَدَّثَنَا ابْنُ أَبِي عُمَرَ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ عُبَيْدِ اللَّهِ، نَحْوَهُ بِمَعْنَاهُ إِلاَّ أَنَّهُ قَالَ قَالَ عُمَرُ بْنُ عَبْدِ الْعَزِيزِ هَذَا حَدُّ مَا بَيْنَ الذُّرِّيَّةِ وَالْمُقَاتِلَةِ ‏.‏ وَلَمْ يَذْكُرْ أَنَّهُ كَتَبَ أَنْ يُفْرَضَ ‏.‏ قَالَ أَبُو عِيسَى حَدِيثُ إِسْحَاقَ بْنِ يُوسُفَ حَدِيثٌ حَسَنٌ صَحِيحٌ غَرِيبٌ مِنْ حَدِيثِ سُفْيَانَ الثَّوْرِيِّ ‏.‏