I am not a Washington lawyer, so you should check with one to be sure. I would think that you are bound by this contract for at least two reasons. 1) It is a contract for a necessity, (shelter), and such contracts are usually enforceable. 2) In order to get out of a contract signed as a minor, you would have had to disavow the contract within a reasonable time after turning 18. It does not sound like this was done.
See here for more information: http://apps.leg.wa.gov/rcw/default.aspx?cite=26.28
Since it appears that you may be stuck with this arrangement, you might try to work out some kind of payment plan with them that you could live with.
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I agree with the previous answer. Another thing to consider is how long it has been since the contract was entered into. Depending on the details, the claim may be barred by the statute of limitations. In addition, depending on the terms of the contract you signed, you may not be liable for the consequence's of your roommate's eviction.
It'd be best to discuss everything with an attorney to decide your best options for resolving this matter as quickly as possible. As noted, you could likely set up a payment plan under a settlement agreement that would mutually beneficial for you and the creditor/landlord.