Volume 3, Book 48, Number 832 :
Narrated by Ibn ‘Umar (radiallaahu `anhu)
Allah’s Apostle (sallallaahu `alayhi wasallam) called me to present myself in front of him on the eve of the battle of Uhud, while I was fourteen years of age at that time, and he did not allow me to take part in that battle, but he called me in front of him on the eve of the battle of the Trench when I was fifteen years old, and he allowed me (to join the battle).” Nafi’ said, “I went to ‘Umar bin ‘Abdul Aziz who was Caliph at that time and related the above narration to him, He said, “This age (fifteen) is the limit between childhood and manhood,” and wrote to his governors to give salaries to those who reached the age of fifteen.
The common conception of a minor being under the age 18 is not in accordance with Islamic practice. Prophet (sallallaahu `alayhi wasallam) let Ibn Umar and some other young companions participate in the Battle of Trench at the age of 15.
With the onset of puberty, the individual becomes subject to shar‘i rulings that apply to him, so he should not shake hands with women or look at them, and he should not be alone with a woman, because he has now become accountable and the things that are prohibited for adult men are also prohibited for him, because he is one of them. It is not permissible to be careless about this matter, and it is essential to take it seriously.
If a fourteen-year-old boy has not yet reached puberty, he is very close to it, and such a person should not be allowed to see women’s ‘awrahs. Women have to understand that and observe hijab in the presence of one who is very close to puberty.
Allah, may He be exalted, says (interpretation of the meaning):
“and not to show off their adornment except only that which is apparent (like palms of hands or one eye or both eyes for necessity to see the way, or outer dress like veil, gloves, head-cover, apron, etc.), and to draw their veils all over Juyubihinna (i.e. their bodies, faces, necks and bosoms, etc.) and not to reveal their adornment except to their husbands, their fathers, their husbands fathers, their sons, their husbands sons, their brothers or their brothers sons, or their sisters sons, or their (Muslim) women (i.e. their sisters in Islam), or the (female) slaves whom their right hands possess, or old male servants who lack vigour, or small children who have no sense of the shame of sex”
Ibn Katheer (may Allah have mercy on him) said: “or small children who have no sense of the shame of sex” that is, because they are so young, they do not understand anything about women and their ‘awrahs, such as their soft speech or alluring ways of walking and moving. If the child is small and does not understand such things, then there is nothing wrong with him entering upon women. But if he is adolescent or close to adolescence, such that he understands these things and can distinguish between who is unattractive and who is beautiful, then he should not be allowed to enter upon women.
End quote from Tafseer Ibn Katheer (6/45-46)
It was said to Imam Ahmad (may Allah have mercy on him): When should a woman cover her head in front of a boy? He said: When he reaches the age of ten years.
End quote from al-Mughni (7/100)
With regard to reaching the age of fifteen years, what is meant is that the boy has completed fourteen years and has entered his fifteenth year. This is often applicable to one who says that he is fourteen years old; he has completed fourteen years. Therefore this sign of puberty is applicable in his case.
Moreover, the years that count in this matter are Hijri years, not Gregorian. This means that his age will be less than if it is counted by Gregorian years.
Taken from IslamQA