Yahya bin Wadh يحيى بن واضح · The Book on Jihad

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.

"Umar said: 'This is the limit that distinguished between children and soldiers.'" And he did not mention him writing about the salary The Hadith of Ishaq bin Yusuf is a Hasan Sahih Gharib Hadith as narration of Sufyan Ath-Thawri