Sayyiduna Abdullah bin Umar (may Allah be pleased with them) says: My participation in the Battle of Uhud was presented before the Messenger of Allah (peace and blessings of Allah be upon him); my age was fourteen years, so I was not granted permission. And in the Battle of Khandaq, my age was fifteen years, so the Messenger of Allah (peace and blessings of Allah be upon him) granted me permission to participate.
Hadith Referenceالسنن المأثورة / باب الجهاد / 627
Hadith Takhrijسنن ترمذی، الاحکام، باب ماجاء فی حد بلوغ الرجل والمرأة، رقم: 1361 وقال حسن صحیح وقال الالبانی صحیح۔