As in hadith below it is explained, why old age is brilliant light on the day of Resurrection? Imam al-Sadiq (AS) said, ‘I have never seen anything catch up with something faster than old age catches up with a believer, for verily it is a source of dignity for the believer in this world and a brilliant light for him on the Day of Resurrection. Allah, Most High, honoured Abraham (AS) with it [old age], who said, ‘What is this, my Lord?’ to which Allah replied, ‘This is dignity’, so Abraham said, ‘O my Lord, increase me in dignity then.’[Amali al-Tusi, p. 699, no. 1492]..
In the religion of Islam, old age has been considered to be a privilege for the aged in order for others to respect them. There are two sets of traditions in this regard one of which emphasizes on respecting aged people under all circumstances without restrictions. The holy Prophet of Islam (S) has been narrated as having said: "Whoever knows the merit and respect of old age and respect the aged ones, God will relieve him of the fear and horror of the Day of Resurrection.
Imam Sadiq (A.S) said, "He is not from us, who does not respect the old and do not have pity on children."
Another set of traditions have also been reported about respecting aged and faithful Muslims. Respecting aged ones who are Muslim and faithful has been stressed upon. The Holy Prophet (S) has been reported as having said: "Whoever respects a Muslim (man or woman) whose hair has turned white due to old age in the way of Islam, God will relieve him of the fear and horror of the Day of Resurrection."
Imam Sadiq (A.S) said: "Respecting a white-haired believer, is a part of respecting the Lord. Whoever respects a believer has begun to respect God and whoever disregards a white-haired believer, God will designate someone to belittle him."
For an old and aged Muslim person especially he who is believing one, in addition to worldly respect accorded to him in the religion, old age has been considered to be a privilege for the aged on the Day of Resurrection. The whiteness of the hair will be for him as brilliant light on the Day of Resurrection.
In this regard, Imam Sadiq, peace be upon him, says: "I have never seen anything catch up with something faster than old age catches up with a believer, for verily it is a source of dignity for the believer in this world and a brilliant light for him on the Day of Resurrection. Allah, Most High, honoured Abraham (AS) with it [old age], who said, ‘What is this, my Lord?’ to which Allah replied, ‘This is dignity’, so Abraham said, ‘O my Lord, increase me in dignity then."
Also, else where Imam Sadiq (A.S) said: "A man went to the Prophet (S) who looked at his hair and said that it was light. He whose hair has become white in Islam, the whiteness of his hair will turn to brightness on the Day of Resurrection."
One of the reasons for the fact that this sign of old age will be brilliant light for the believer is that an aged man has had more opportunity to worship God and attain the degrees of faith (iman) and also to get rid of the darkness of ignorance and nafs (carnal soul). Since he has availed himself of the opportunity acquiring the light of faith, as evidenced by many verses of the Holy Quran and that God himself places the divine light in a believer's heart, and the believer had been increasing the brilliance of his body and soul, God the Exalted will make his hair a sign of the brilliant light of his soul, the light which he has gained as a result of worshipping Allah and acquiring some degrees of faith.
 .«مَنْ عَرَفَ فَضْلَ کَبِیرٍ لِسِنِّهِ فَوَقَّرَهُ آمَنَهُ اللَّهُ مِنْ فَزَعِ یَوْمِ الْقِیَامَةِ» Kulayni, Muhammad, Al-Kafi, researched and edited by Ghaffari, Ali Akbar, Akhundi, Muhammad, vol.2, p. 658, Dar al-Kutub al-Islamiyah, Tehran, fourth edition, 1407 A.H.
 «لَیْسَ مِنَّا مَنْ لَمْ یُوَقِّرْ کَبِیرَنَا، وَ یَرْحَمْ صَغِیرَنَا» Sheikh Hurr Amili, Muhammad bin Hasan, Hidayat al-Ummah Ila Ahkam al-Aemmah (A), vol.5, p. 156, Astaan al-Razawiyah al-Muqaddash, Majma' al-Bohuth al-Islamiyah, Mashad, first edition, 1414 A.H.
 «مَنْ وَقَّرَ ذَا شَیْبَةٍ فِی الْإِسْلَامِ آمَنَهُ اللَّهُ عَزَّ وَ جَلّ َمِن ْفَزَعِ یَوْمِ الْقِیَامَةِ» Al-Kafi, vol.2, p. 658.
 «مِنْ إِجْلَالِ اللَّهِ عَزَّ وَ جَلَّ إِجْلَالُ الْمُؤْمِنِ ذِی الشَّیْبَةِ وَ مَنْ أَکْرَمَ مُؤْمِناً فَبِکَرَامَةِ اللَّهِ بَدَأَ وَ مَنِ اسْتَخَفَّ بِمُؤْمِنٍ ذِی شَیْبَةٍ أَرْسَلَ اللَّهُ إِلَیْهِ مَنْ یَسْتَخِفُّ بِهِ قَبْلَ مَوْتِهِ»، Al-Kafi, vol.2, p. 658.
 Sheikh Tusi, Al-Amali, al-Nas, p. 699, Dar al-Thaqafah, Qom, first edition, 1414 A.H.
 Al-Kafi, vol.6, p. 480.
 "And to whomsoever Allah does not give light, he has no light." [al-Noor, 40] "That is Allah's grace; He grants it to whom He pleases, and Allah is the Lord of mighty grace." [al-Jumu'ah, 4] "He is the one who follows the right way; and whomsoever He causes to err, these are the losers." [al-A'raf, 178] "his is Allah's guidance, He guides thereby whom He pleases of His servants." [ al-An'am, 88]
 Sadruddin Shirazi, Muhammad, Sharh Usool al-Kafi, researched and edited, Khajawi, Muhammad, vol.1, p. 207, Center for Cultural Research and Studies, Tehran, first edition, 1383 A.H