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 reviewed him on the day of the Trench when he was fifteen and permitted him.
أن النبي ﷺ عرضه يوم أحد، وهو ابن أربع عشرة فلم يجزه، وعرضه يوم الخندق وهو ابن خمس عشرة فأجازه.
Bukhari narrated it in Maghazi (4097) and Muslim in Imarah (1868:91), both through the route of Ubaydullah (he is Ibn Umar) from Nafi, from Ibn Umar, who said, and he mentioned it.