Even though your 18 year old son is no longer a minor, if he is driving your vehicle with your permission, you will still be held accountable. This is called 'vicarious liability.' As the driver, your son's insurance is 'primary.' After his limits are exhausted, the other party can come after your insurance company for compensation. Now, you can click here to help you with California law regarding how much you are responsible. However, based on the way it reads, you may need to seek legal advice to explain it to you. That being said, I do not believe it says your liability will be no more that $15,000.