Sayyiduna Ibn Umar (may Allah be pleased with them both) said: I was presented before the Messenger of Allah (peace and blessings be upon him) on the day of Uhud when I was fourteen years old, and at that age he (peace and blessings be upon him) considered me too young. And on the day of the Trench (Khandaq) I was presented before the Messenger of Allah (peace and blessings be upon him), when I was fifteen years old, and at that age he (peace and blessings be upon him) accepted me.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السرقة / 17207
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
It is narrated from Qasim (may Allah have mercy on him) that a slave girl who had stolen and who had not reached puberty was brought to Sayyiduna Abdullah (may Allah be pleased with him), and he did not cut her hand.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السرقة / 17209