Answer: In the name of Allah, the Most Gracious, the Most Merciful.
Hanafi School: At the age of 10.
Maliki School: At the age of 11, as I understood from some of their works.
Shafi School: At all ages.
Hanbali School: At the age of 10.
I mention these opinions to correct a common misconception among parents whose children haven’t reached the age of puberty that they are permitted to let them dress without a Hijab until they reach the age of puberty. If puberty occurs before these ages then children are addressed with all obligations, including the Hijab, as are their parents. However, if it delays above these ages then they are required to adapt to the Muslim dress code. To facilitate this adjustment, parents should ease their children into the practice of Hijab so it is easier for them to adjust.
My personal recommendation would be to aim to accomplish the goal of training girls to wear Hijab before the age of 10, which is consistent with the Hanafi & Hanbali school. The Shafi position on this issue seems to be a mere academic stance and is not practiced by all of the Shafi scholars I’ve asked. The Maliki school is the most lenient in this regard, but as Dr. Wahbah al-Zuhaily put it, “It appears to me that this position (Hanbali position) and that of the Hanafis is more appropriate because of it’s consistency with the tradition in which [the Prophet commanded for the parents] to command their children to pray at the age of 7 and discipline them to pray at the age of 10.”
And Allah knows best.
Abdul Wahab Saleem