Ash-Shaykh Muhammad bin Saalih al-Uthaymeen rahimahullah was asked the following question :
متى تحتجب المرأة من الصغير؟
When does a woman wear a Hijaab from a young boy?
لم يقيد الله -عز وجل- ذلك بسنٍ معين، وإنما قال: أَوِ الطِّفْلِ ﴿الَّذِينَ لَمْ يَظْهَرُوا عَلَى عَوْرَاتِ النِّسَاءِ﴾ [النور:31] فإذا كان هذا الطفل لا يأبه بالمرأة، ولا يلتفت إليها، ولا يذكر أوصافها فإنه لا حرج عليها أن تكشف وجهها عنده، أما إذا علم من نظراته وتتبعه النساء أن فيه شهوة؛ فإنه يكون هنا قد اطلع على عورات النساء فلا يحل للمرأة أن تكشف أمامه، وفي الغالب أن من بلغ عشر سنوات فإنه يطلع على عورات النساء.
Allah hasn’t restricted that to a certain age rather, He only said :
﴿الَّذِينَ لَمْ يَظْهَرُواْ عَلَى عَوْرَتِ النِّسَآءِ﴾
(Those who have no awareness of the hidden aspects of women)
Therefore, if this child gives no concern to women nor does he give them attention and doesn’t mention their description (to others) then there is no sin upon her to uncover her face during his presence. As for if it is known by his looks and by him giving much attention to women that he has a desire (for them) then (i.e that is a sign) that he has indeed obtained the awareness of the hidden aspects of women, so it is not allowed for a woman to uncover herself in front of him. And in most cases whosoever reaches ten years old will possesses with him such awareness regarding women.”
An Excerpt of The English Translation of Tafseer Ibn Katheer pertaining to the verse:
﴿أَوِ الطِّفْلِ الَّذِينَ لَمْ يَظْهَرُواْ عَلَى عَوْرَتِ النِّسَآءِ﴾
(or children who are not aware of the hidden aspects of women. ) Because they are so young they do not understand anything about women or their `Awrah or their soft speech or their enticing ways of walking and moving. If a child is small and does not understand that, there is nothing wrong with him entering upon women, but if he is an adolescent or approaching adolescence, so that he knows and understands these things, and can make a distinction between who is beautiful and who is not, then he should not enter upon women. It was recorded in the Two Sahihs that the Messenger of Allah said:
«إِيَّاكُمْ وَالدُّخُولَ عَلَى النِّسَاءِ»
(Avoid entering upon women.)
Source : Tafseer ibn Katheer.
Also there is another verdict by Shaykh Bin Baaz rahimahullah that it is only an obligation upon the woman to wear the hijaab in front of the boys who have reached puberty and she may cover herself cautiously in front of those who are adolescent or approaching adolescence.
[Reference removed since it no longer exists]
AbdulFattaah bin Uthman