In WA, a person becomes a legal adult at age 18 unless the person has been found by the court to be incompetent.
So, you can leave on your 18th birthday. While you are legally subject to your mother's control while you are under 18, as a practical matter, the police likely is not going to do anything about an older teen running away if the teen voluntarily leaves and does not appear to be in any immediate danger (such as being prostituted, drugged, etc.).
However, before you start running away, have you thought about practical things such as where will you be living? how will you pay rent? where's the money to buy food?
You can leave when you turn 18 if that is your wish.
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