What is the age at which a woman should observe hijab in front of a boy?

Allaah says in the passage
where He speaks of those to whom it is permissible to show one’s adornments
(interpretation of the meaning): 

“…or small children who
have no sense of feminine sex”

[al-Noor 24:31]

If a child shows some
awareness of a woman’s ‘awrah and starts to look at her and talk to her a
great deal, then it is not permissible for a woman to uncover in front of
him. 

This varies from one boy to
another in terms of natural disposition and in terms of the company that he
keeps. A boy may have a greater interest in women if he sits with people who
talk about them a great deal, and if it were not for that he would not be
particularly interested in them.  

What matters is that Allaah
has set the guidelines for this matter when He said (interpretation of the
meaning): 

“…or small children who
have no sense of feminine sex”

[al-Noor 24:31]

i.e., these are among the
ones in front of whom it is permissible for a woman to show her adornments,
if they have no interest in women. 

Shaykh Ibn ‘Uthaymeen
(may Allaah have mercy on him), Majmoo’at As’ilah tahumm al-Usrah
al-Muslimah, p. 148

Among the things which indicate that a child has started to
be aware of a woman’s ‘awrah are the following:

-He describes women to others

-He can distinguish between a
beautiful woman and an ugly one

-He compares the way women look

-He stares at women for a long
time

Undoubtedly movies and soap operas, as well as social corruption, lead to children being aware of women’s ‘awrahs at an early age, so we have to be very careful.  We ask Allaah to keep us safe and sound.