What age?

St

My daughter is 9 and asked too go to the park with her 2 friends. I said "sure let me get my shoes". She then says "No,without you,by myself.Their parents don't come".So,I tell her ok."Take your phone and when the timer goes off (20min) You need to come home. " So,I watch her leave too the park from my Window until I can no longer see her.Rush,get my shoes on and sneak too the park and sit where she can't see me.What age is an appropriate age too allow them to go too the park with only friends? At my old house I could hear and see her at the park from my back window. At our new house the park is 5min from my house and I can't see or hear her. It worries me that the other parents allow their kids too go,age 7,8,9.The park bench is where I was sitting...When I heard her timer go off I ran home and acted like I was their the whole time😂😂😂