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

About age 6-7 i reckon. That's about the right age range for them to both start exploring their fearful feelings, and to be able to separate fact from fiction. Probably why Goosebumps and similar stories are so popular for that age group. Some children are fine with all sorts of stuff, and others have difficulty sleeping at night because their imagination gets the better of them. A lot of it will depend on your child's maturity.