From Sahih Bukhari (Hadith 2664):
Narrated Ibn `Umar:Allah's Messenger (ﷺ) called me to present myself in front of him on 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 bin `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
Reference: Bukhari 2664