How old were your children when you let them shower alone unassisted?

My step daughter is 10, I stay in there with her long enough to make sure all the shampoo and conditioner are out of her hair.(because her hair is very long and curly) Her mother let's her shower at her house by her self with no assistance and she is always coming to our house with her hair crusty from not washing it properly? Mom says she can do it but clearly she can't!? How would you approach the issue?