> The way you know it's magic is it shatters the principle of referential identity, which tells you that a variable is itself

When you do a useValue() it is clear you are grasping at something with a larger lifespan than your function.

Conceptually it is not much different than

    int counter() {
      static int x = 1;
      return x++;
    }

Which predates C89, if age is an argument