I'm not a West Virginia lawyer, but I'd be pretty certain that the answer is 18, when the child is legally an adult and can live wherever they choose.
In most states, a child who is old enough (and that can vary, depending on the child's intelligence and maturity) will be able to say what they want, but it won't be the only thing the court looks at, and it isn't necessarily how the decision will come down. I'd say the child would have to be at least 7 or 8, to be heard; the older the child, the more weight is usually given to their wishes.
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