In MO, insurance companies have historically looked at the age of the kids to determine liability...a 7 year old couldn't form intent, but a 13 year old should know better. There are no hard and fast rules. Also in MO, debris coming off your vehicle is your responsibility, so if a truck is hauling gravel and it hits cars, there is liability there, but if you run over something on the road and your vehicle kicks it up on another car, you are not liable.

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2002 Ford ZX2
2 x 2013 Honda Civics