Umar bin Abd al-Aziz (may Allah have mercy on him) said: This is the dividing line between the young and the grown-up.
As for looking beneath the lower garment (izar) and taking the growth of hair as evidence of puberty, the narrations about it have already passed in the Book of Interdiction (Kitab al-Hajr).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السرقة / 17208