The Prophet ﷺ reviewed him on the day of Uhud when he was fourteen years old and did not permit him to take part. Then he reviewed him on the day of the Trench when he was fifteen, and permitted him.
حدثنا يحيى عن عبيد الله أخبرني نافع
أن النبي ﷺ عرضه يوم أحد وهو ابن أربع عشرة فلم يجزه ثم عرضه يوم الخندق وهو ابن خمس عشرة فأجازه
Its isnad is sahih, and Bukhari and Muslim also narrated it, as mentioned in Ibn Kathir's history 4:15. Tirmidhi narrated it with two isnads through Ubaydullah 2:288, then repeated it with the same two isnads 3:35, and he said, "The hadith is hasan sahih."