You certainly raise an interesting scenario! If I understand your question correctly, you were married when you were under the age of 18 and your spouse incurred a debt. Now the collector is trying to collect from you because your spouse (ex-spouse?) has not paid. Is that correct?
You indicate you live in Washington state and I assume that's where you lived when you were married. If so, Washington is a community property state which may mean that debts incurred by you or your spouse after your marriage are considered community property. That could mean you would be on the hook for debts she incurred while you were married.
However, you also mentioned that you were a minor at the time. Typically, contracts with minors cannot be enforced. That could mean you are off the hook.
Because this is a complicated situation, I would recommend you talk with a consumer law attorney who can explain your rights. Call the Collection Complaint Hotline at 888-711-5143 for a free, confidential case evaluation.