The Messenger of Allah ﷺ reviewed him on the day of Uhud when he was fourteen years old and did not permit him to join the battle. He reviewed him again on the day of the Trench when he was fifteen years old and permitted him.
أخبرنا عبيد الله بن سعيد قال حدثنا يحيى عن عبيد الله قال أخبرني نافع
أن رسول الله ﷺ عرضه يوم أحد وهو ابن أربع عشرة سنة فلم يجزه وعرضه يوم الخندق وهو ابن خمس عشرة سنة فأجازه
Bukhari, chapter on the Battle of Khandaq (which is Ahzab), in the Book of Military Expeditions, no. 4097, from the hadith of Yahya bin Saeed Qattan, with the same chain, and it is in Kubra, no. 5624.