The covering becomes obligatory at puberty (bulūgh) — marked by menstruation or the other recognised signs — because that is when a person becomes religiously accountable; before that a girl is not sinful for uncovering. The Prophet ﷺ said, "Allah does not accept the prayer of a woman who has reached puberty except with a khimār" (Sunan Abī Dāwūd 641), and Allah commands the believing women to draw their khimārs over their chests and to draw their jilbābs over themselves. That said, the prophetic method of upbringing is habituation before obligation — just as children are told to pray at seven, years before it is binding — so many scholars recommend that a girl be gently accustomed to covering in the years approaching puberty. What is counter-productive is imposing it on an older girl with no explanation; teach the "why" (dignity, obedience to Allah, worship) alongside the "what", and make the home a place where she is praised for it rather than shamed into it.
Q&A · Youth & Parenting
At what age must a girl start wearing hijab?
Evidence — the daleel
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Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.