At what age can a child have a say in where she lives?

At what age can a child have a say in where she lives? My stepdaughter, who is 8, cannot stand her stepfather and she does not want to be around him at her mother's house. He is not physically abusive (although he did go to jail for domestic violence in 2005), but is mentally and emotionally abusive. We currently have 40% custody and her mother has 60%, although essentially her time is split between the households 50/50. There is no stability at her mother's house and she says she feels like a second-class citizen. None of this is physical and is difficult to prove.