Abdullah bin Umar reported,

The Messenger of Allah ﷺ reviewed me on the day of Uhud, when I was fourteen years old, and did not permit me to join the fighters. Then he reviewed me on the day of the Trench, when I was fifteen years old, and permitted me.Nafi said, I came to Umar bin Abdul-Aziz when he was caliph and reported this hadith to him. He said, "This is indeed the boundary between the minor and the adult," and he wrote to his governors instructing them to assign a stipend to anyone who had reached the age of fifteen.

Bukhari 2664

Sahih

حدثنا عبيد الله بن سعيد حدثنا أبو أسامة قال حدثني عبيد الله قال حدثني نافع قال حدثني

ابن عمر رضي الله عنهما

أن رسول الله ﷺ 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

البخاري 2664صحيح
Contents
Reference
Commentary
Ayat
Collection
Sahih Bukhari
Book
Testimonies
Chapter
Puberty of children and their testimony
Numbering
Darussalam 2664
Bukhari 2664