Can I sue an 18-year-old for something that happened when she was 17?

Yes you may.

I do not understand the facts here which are critical. Are you actually the adoptive mother of this child, or is she a stepdaughter as child of your current husband prior to the commencement of your relationship or are you simply living together and consider her to have that relationship? Your facts state she actually stole your car in which case you should be covered by insurance, and she should be prosecuted do to the fact that there is been some very significant damages. That might potentially cause her to be sentenced as liable for restitution, in addition to her other punishments.

No. The event occurred when she was a minor.

No, you cannot sue her if she was a minor at the time of the accident. You will have to sue her guardian, which could be yourself. I suggest you work something out with the bank, or file bankruptcy to get rid of the debt.

You are describing a tort, and torts are outside my scope. State law varies for a minor's liability. A 17-year-old may be liable, and the parents may also be jointly liable. Provided you did not authorize her to drive the vehicle, your real dispute is with the insurance company. The insurer's liability depends upon conditions in the insurance contract. Consult with an attorney (no fee initial consultation) to explore remedies against the insurance company and your step-daughter.