It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) called him forward to fight on the day of Uhud. Ibn Umar (may Allah be pleased with them both) said: I was fourteen years old, so he (peace and blessings be upon him) did not permit me. Then he called me forward on the day of al-Khandaq, when I was fifteen years old, and he (peace and blessings be upon him) permitted me. Nafi' (may Allah have mercy on him) said: I came to Umar bin Abd al-Aziz (may Allah have mercy on him) when he was caliph, and I narrated this hadith to him. He said: This is the dividing line between the young and the adult. Then he wrote to his governors to allot a share for whoever had reached fifteen years, and whoever was below that, to include him with the household.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 13004
Zayd bin Aslam narrated from his father that Umar (may Allah be pleased with him) said: Gather this fay' wealth so that we may look into it. Then later he said to them: I had commanded you to gather it so that we may look into it, and I have read verses in the Book of Allah and found them sufficient. Allah says: "What Allah has given as fay' to His Messenger from the people of the towns belongs to Allah and to the Messenger" [Surah al-Hashr: 7] up to His words "severe in punishment" [Surah al-Hashr: 7]. By Allah, it is not for these alone. Then he read: "For the poor emigrants who were driven out of their homes and their properties" [Surah al-Hashr: 8]. Then he read: "And those who came after them say: Our Lord, forgive us and our brothers who preceded us in faith" [Surah al-Hashr: 10]. By Allah, it is not for these alone either. And if I remain until next year, I will surely join the last of the people with the first of them, and I will surely make them all one single category. While he was distributing, a son of his came, Abd al-Rahman bin Luhayyah, from one of the wives of Umar (may Allah be pleased with him), and said: Give me a ring to wear. Umar (may Allah be pleased with him) said to him: Go to your mother; she will give you a drink of sawiq (barley gruel). By Allah, he did not give him anything.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 13005