Sahih Al Bukhari (657)

حدثني ابن عمر رضي الله عنهما "أن رسول الله (صلى الله عليه وسلم) عرضه يوم أحد وهو ابن أربع عشرة سنة فلم يجزني ثم عرضني يوم الخندق وأنا ابن خمس عشرة سنة فأجازني قال نافع فقدمت على عمر بن عبد العزيز وهو خليفة فحدثته هذا الحديث فقال إن هذا لحد بين الصغير والكبير وكتب إلى عماله أن يفرضوا لمن بلغ خمس عشرة"

Ibn `Umar narrated: Allah's Messenger (peace be upon him) called me to present myself in front of him or the eve of the battle of Uhud, while I was fourteen years of age at that time, and he did not allow me to take part in that battle, but he called me in front of him on the eve of the battle of the Trench when I was fifteen years old, and he allowed me (to join the battle)." Nafi' said: "I went to `Umar ibn 'Abdul ‘Aziz who was Caliph at that time and related the above narration to him. He said: "This age (fifteen) is the limit between childhood and manhood," and wrote to his governors to give salaries to those who reached the age of fifteen.