What is the appropriate age to let your children walk around the mall alone?

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As most of you already know, my daughter is going to be 11 in February. Most of her friends walk around in the mall by themselves. I let her go to the mall with one of her friends after checking to make sure they would have adult supervision at all times to find out later that they were dropped off and told to call when they were ready to come home. I was outraged. Am I being an overly protective mom or is it normal to let kids this age walk around by themselves? What are your thoughts?