It depends on the child. I think the time for a child to have a pocket knife is when he/she has a use for one. From memory, I bought my daughter a Spyderco with the Spider pattern cut into the blade for her birthday when she was about thirteen, mostly because she was into drawing spider web designs.
I have an assortment of pocket knives and to be honest, I can't remember the last time I ever needed to use one to cut anything. I think there is a Leatherman in the glovebox of my car.
Swiss Army knives are useful for the scissors and the tweezers.