The number of people who have gray hair is very high among residents living in florida. a student concludes that living in florida causes hair to turn gray. what is the flaw in this student's reasoning?

The student used correlation not causation. There is a high correlation with gray haired residents and Florida. But the cause of graying hair is not living in Florida. It might be something totally different. It might be a coincidence that it so happens to be in Florida. The student needs more evidence to make such a leap in logic.