At what age are Muslim girls expected to start wearing the hijab?

Answer:
In Islam, all individuals become responsible and accountable for their behavior at the age of puberty.
The Islamic teachings about modest dress apply to any person, male or female, at that time.
Islamic teachings about modest clothing are based on the Qur'an, and on the specific sayings/traditions of the Prophet Muhammad, known as the hadith. In one often-cited tradition, it is reported that a young woman visited Muhammad's wife Aisha while she was wearing see-through clothing. Muhammad averted his eyes and told her, "After a young woman reaches the age of puberty, nothing should be seen of her except this and this," motioning to his face and hands.
Therefore, it is expected at this age for Muslim girls to adapt more modest styles of dress that do not expose too much.

Even before the age of puberty, many girls become accustomed to these values of modesty. Muslim parents are generally careful about choosing appropriate clothing, even for their young children. While they may or may not wear the hijab yet, many young girls choose not to wear short skirts or shorts, belly shirts, or sleeveless tops in public.

In some families and cultures, girls begin to wear the hijab at puberty simply out of family, peer, or societal pressure. Some girls decide, on their own, to start wearing the hijab even before the age when it is required of them. It is becoming more common for families to leave the decision about hijab up to the young woman herself. They may try to educate her about her Islamic responsibilities, but ultimately they allow her to make the choice when she fully understands and feels ready to commit to the decision.

It is important to note that Islamic values of modesty apply to both men and women, and that the responsibility for making good choices in this regard falls on boys and girls equally.