At what age is it okay to tell ghost stories to kids? Is it healthy to, though?

When kids read, they can lose themselves in imaginary worlds Narnia? Hogwarts? Camp Half-Blood? Seriously, those places do not exist, and pretending they do is just plain silly. Will knowing where to place them on an imaginary map help my kid with his geography grade? Will reading about how to train a dragon get my boys to walk their dog more often? I think not. Why let them dream about far-away, fairytale lands when real life is always so wonderful?