can a 16 year old child decide which parent they want to live with under WA state child custody laws

In establishing the parenting plan, the court may consider "the wishes of a child who is sufficiently mature to express reasoned and independent preferences as to his or her residential schedule". See RCW 26.09.187 (Criteria for establishing permanent parenting plan). The statutes are here: http://apps.leg.wa.gov/RCW/default.aspx?cite=26.09 .

However, the wishes of the child is just one factor for the court to consider. The more mature the child is, the likelier the court will consider the child's wishes.

Once a child is a legal adult at 18, then the child will have an absolute right in determining where the child lives.