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
Volume 3, Book 29, Number 80:
Narrated Ibn ‘Abbas (radiallaahu `anhu):
The Prophet (sallallaahu `alayhi wasallam) sent me (to Mina) with the luggage from Jam’ (i.e. Al-Muzdalifa) at night.
Volume 3, Book 29, Number 81:
Narrated ‘Abdullah bin ‘Abbas (radiallaahu `anhu):
I came riding on my she-ass and had (just) then attained the age of puberty. Allah’s Apostle (sallallaahu `alayhi wasallam) was praying at Mina. I passed in front of a part of the first row and then dismounted from it, and the animal started grazing. I aligned with the people behind Allah’s Apostle (sallallaahu `alayhi wasallam) (The sub-narrator added that happened in Mina during the Prophet’s Hajjat-ul-wada.)
Volume 3, Book 29, Number 82:
Narrated As-Sa’ib bin Yazid (radiallaahu `anhu):
(While in the company of my parents) I was made to perform Hajj with Allah’s Apostle (sallallaahu `alayhi wasallam) and I was a seven-year-old boy then.
Volume 3, Book 29, Number 83:
Narrated Al-Ju’aid bin ‘AbdurRahman:
I heard ‘Umar bin ‘Abdul Aziz telling about As-Sa’ib bin Yazid that he had performed Hajj (while carried) with the belongings of the Prophet (sallallaahu `alayhi wasallam).
Children who have not yet reached the age of puberty will be rewarded for the good deeds they do. Ibn ‘Abbaas (may Allaah be pleased with them both), said: “A woman lifted up a child and said, ‘O Messenger of Allaah, will his Hajj (pilgrimage) be counted?’ He said, ‘Yes, and you will be rewarded.’” [Muslim]
The author of Mawaahib al-Jaleel said, concerning children entering ihraam (sacred state) for Hajj and ‘Umrah:
“The scholars do not differ concerning the fact that children will be rewarded for whatever acts of obedience (to Allaah) they do, and will be let off for any bad deeds that they do, and any bad deeds they do deliberately will be counted as mistakes. It says in Mukhtasar al-Waadihah: ‘Hajj is not an obligation for boys and girls until boys reach puberty and girls begin menstruating, but there is nothing wrong with taking them for Hajj. It is recommended (mustahabb), and the Messenger of Allaah (peace and blessings of Allaah be upon him) did so.’”
Then he reported that Talhah ibn Musarrif said: “It was one of the customs of the Muslims to take their children for Hajj and expose them to the mercy of Allaah.”
Ibn ‘Abd al-Barr said in al-Tamheed that it is encouraged to take children for Hajj, and that the majority of scholars say this. He also said: “It is nothing strange that a child should earn reward and status in the Hereafter for his salaah (prayers), zakaah (paying poor-due), Hajj and other good deeds if he does them as they should be done, (because this is) grace and mercy from Allaah, just as Allaah shows mercy to the dead by rewarding them for acts of charity done on their behalf by the living. Do you not see that they (the scholars) are agreed that a child should be commanded to pray when he reaches the age of understanding and that the Prophet (peace and blessings of Allaah be upon him) led Anas and the orphan in prayer? Most of the salaf (pious predecessors) said that zakaah has to be paid on the property of orphans, and it is impossible that they would not be rewarded for that. Their guardians and the one who does this on their behalf will also certainly be rewarded, as will the one who takes them for Hajj, as a blessing and mercy from Allaah. It was reported that ‘Umar said: ‘Children’s good deeds will be recorded and their bad deeds will not be recorded.’ I do not know of anyone whose opinion is worth following who said anything to the contrary.”
More on it here.