I'm really wondering, what is the legal age to go into a relationship(either sexual or just as boyfriend/girlfriend relationship) with for a 17 year old in the State of Washington?

You can date anyone you please. The age of consent for sex appears to be 16.

Now if you are asking if it's a good idea or not...

Keep in mind that a minors parents may forbid such a relationship and thsu intheory make it illegal

I am interested in your research - a person CAN consent to sex at 16 in Washington and is of majority/legal age at 18 in Washington. Whether the parents forbid it the parties (at 18) are of legal age.

I don't understand why you think this, in theory, would be an illegal relationship.

I'm not saying that parents shouldn't and can't "control" the children, of age or not, who live with them - I'm saying I see no illegal activity.

Again - please post your research.

While the age of consent is 16, there are side issues like the age difference ans relationship. For example a teacher over 23 having sex with a 17 yr old would probably be considered illegal.