"We were presented to the Messenger of Allah (ﷺ) on the day of (the battle of) Quraizah. Whoever had pubic hair was killed and whoever did not was left to his way. I was of those who did not have pubic hair so I was left to my way." [Abu 'Eisa said:] This Hadith is Hasan Sahih. This is acted upon according to some of the people of knowledge. They consider pubic hair an indication of the age of responsibility, if it is not known whether he has had a wet dream, or his age. This is the view of Ahmad and Ishaq.
حَدَّثَنَا هَنَّادٌ، حَدَّثَنَا وَكِيعٌ، عَنْ سُفْيَانَ، عَنْ عَبْدِ الْمَلِكِ بْنِ عُمَيْرٍ، عَنْ عَطِيَّةَ الْقُرَظِيِّ، قَالَ عُرِضْنَا عَلَى النَّبِيِّ صلى الله عليه وسلم يَوْمَ قُرَيْظَةَ فَكَانَ مَنْ أَنْبَتَ قُتِلَ وَمَنْ لَمْ يُنْبِتْ خُلِّيَ سَبِيلُهُ فَكُنْتُ مِمَّنْ لَمْ يُنْبِتْ فَخُلِّيَ سَبِيلِي . قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ . وَالْعَمَلُ عَلَى هَذَا عِنْدَ بَعْضِ أَهْلِ الْعِلْمِ أَنَّهُمْ يَرَوْنَ الإِنْبَاتَ بُلُوغًا إِنْ لَمْ يُعْرَفِ احْتِلاَمُهُ وَلاَ سِنُّهُ وَهُوَ قَوْلُ أَحْمَدَ وَإِسْحَاقَ .
Tirmizi
The Book on Military Expeditions • Hadith 1628