Abdullah bin Muslima al-Qa'nabi عبد الله بن مسلمة بن قعنب القعنبي · The Book of Divorce

It was narrated from Ibn 'Umar that he presented himself to the Messenger of Allah on the Day of Uhud when he was fourteen years old, but he did not permit him (to join the army). He presented himself on the Day of Al-Khandaq when he was fifteen years old, and he permitted him (to join the army).