Dear Sir/Madam,
Under the contract law, a person under the age of 18 years old lacks the capacity to contract except contract providing necessities of incapacitated party. However, minor can affirm after attaining majority. But If contract between minor and adult, It is binding on adult. So If you signed a contract for car with adult, he can not back off because it's binding on him. Therefore, he cant take the car away from you. You can re-enforce the contract through parent or guardian and ask for damages or specific performance (getting your car back).
If you have any other question, please feel free to contact me. Please dint forget to accept the answer. A bonus and positive feedback is always welcome.
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