At what age do Muslim women start wearing head scarves?

A coworker is Muslim. His wife chooses to cover her hair in a wrap. He has two young boys and a baby daughter. The daughter just turned 1. Her hair isn't covered. Is there a certain age when a female begins to cover her hair?

My coworker is very open and eager to educate but I still think this is a weird question to ask him lol
At what age do Muslim women start wearing head scarves?
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