I am an expatriate 15-year-old girl. This summer, I traveled to my home country to spend some time with my family members with whom we regularly gather. I am veiled (wearing the face cover) and I even used to wear the veil before young boys. However, I once sat with my hair uncovered while wearing pants in front of my 11-year-old cousin's son. Is this permissible? When am I required to start covering my face before young boys? What type of boys am I permitted to sit in front without wearing the veil, sufficing with Hijab and other properly covering clothes?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

You should know that a mature girl is required to cover all her body (except the face and palms according to some scholars who allow uncovering them) before non-mahram (permanently unmarriageable) men. Scholars, however, provided different opinions regarding the age of non-mahram boys who have not yet reached puberty before whom a woman is required to wear the Hijab. Some are of the opinion that that age is 10 years old. Ibn Qudamah said, "It was said to Abu ‘Abdullah (i.e. Imam Ahmad), 'When is a woman required to cover her head before a boy?' He answered, 'When he is ten years old.'" [Ash-Shar-h Al-Kabeer]

Thus, it is more cautious that you do not sit before this boy while uncovering your hair. As for mahram (permanently unmarriageable) boys, regardless of how old they are, you are permitted to uncover before them what usually appears of the woman's body, such as the head, palms and neck. Mahram men are those male kindred who are permanently unmarriageable to you, such as a brother, nephew, paternal or maternal uncle. 

Allah knows best.