If a car title has both the parent's and adult child's name on it, the parent can legally take the car away from the adult child, even if the child is over 18. This is because the parent is a co-owner of the vehicle, and as such, they have equal rights to the car. In most states, when multiple names are listed on a car title, any action regarding the title or ownership of the car would require both signatures. However, if the title lists the names as or, then either party can act independently regarding the title and ownership of the car.
It's important to note that even if the adult child is over 18, they may still be considered a dependent on their parent's insurance policy. In this case, the parent may have the right to take the car away if they are the primary policyholder and are paying for the insurance. However, this would depend on the specific circumstances and the terms of the insurance policy. Ultimately, it's best to consult with a legal professional or an insurance expert to determine the specific rights and responsibilities of each party in this situation.