When am I obligated to cover my hair from a boy whose puberty status I am ignorant of?

If a child does not comprehend the meaning of 'awrah (intimate parts), then a woman is not obliged to observe hijab in front of him, as Allah Almighty says: "Or [before] children who have not yet become aware of the private aspects of women." However, if he comprehends and can distinguish, then she must observe hijab in front of him. It has been reported from Imam Ahmad that hijab is obligatory in front of anyone who has reached ten years of age, and this becomes more emphatic as he approaches puberty.