Most insurance companies require you to list all of your household residents and so an 18 year old child still living with you would normally need to be listed on your car insurance policy.

With those who are licensed (or of driving age) in your household typically your insurance company will give you the choice of listing them as 'eligible' to drive or 'excluded' from driving. If they are listed as 'eligible,' they will be rated against your policy's premiums. If they are 'excluded,' they will not be rated against your policy's premiums; however, you are accepting full responsibility if you let them drive since your insurance company will not extend them coverage. Even once and no matter the circumstance and get in an accident it would be your responsibility to pay since your insurer would not if they are an excluded driver from your policy.

Now, the example you gave me is an entirely different issue. Besides household residents, your insurance company wants to know who will be driving your vehicle with any regularity or frequency. If someone is going to be driving your vehicle with any regularity or frequency, that person will need to be listed as an 'eligible' driver on your policy. The insurance company will cover someone who does not reside with you and you let the person drive it for a 'one-time' thing.

Someone living in your household has access to your vehicle and could drive it daily and thus is a risk factor your insurer is allowed, by state laws, to use to determine the auto insurance rates for your policy. A friend that you loan you car to from time to time and without any regularity is not a constant driver and thus not a risk factor that normally your insurer would take into account. If however that friend used your vehicle on a regular basis you would need to inform your insurer who may require you to add him or her as a driver on the policy.

Remember, you can get a quote for auto insurance rates by clicking here.