The age at which the child can make this decision in Washington State is 18. The grandparents should seek the help of an attorney. There is no information given here about the mother and about why the daughter has been living with the grandparents all along, but depending on the facts of the case, the grandparents may be able to establish that third party custody is in the best interests of the child. There is a very high bar for this, because the rights of parents to raise their children are not permanently taken away lightly. But if the facts of the case are compelling, it is worth trying.