It is not normally a state law that requires that you place your teenager on your policy when they become licensed but usually a requirement of your insurance company that you inform them of the any licensed household members so that they can rate you appropriately for the risk of the drivers in the household they are covering. The law requires all licensed drivers in a household be listed or indicated in most states.

Your son's friend's parents likely did not inform their insurance carriers of their teens licensing status which could cause problems if their child is in an accident and needs to use the insurance coverages. The insurance company may cover the accident and then make the parents add the child to the policy at that point or even require them to pay back the premiums that they should have previously paid for the child to be covered by the policy. Or the if the policy terms and insurance guidelines allow the insurance cover may not cover the accident since the parents failed to notify them of this licensed household member and add him or her to the policy.

You can check with the Minnesota insurance regulator to find out if there is a law that mandates your child is added to your policy, we are not aware of such a MN state law though. However as we explained it is normally a requirement of your insurance company and not the state which mandates you notifying them of any licensed household members and add them to the policy if the insurance company guidelines require you to do so.