... If you are under 18, you can revoke the purchase.
Although in MOST cases those under the age of 18 cannot be held to contracts that they sign, this is not true in ALL cases. It depends on the facts. For a car purchased under circumstances where the car can be considered a "necessity" (like food or clothing), then being a minor may not be an acceptable excuse for cancelling the contract. Not that this is always easy to prove (or worth proving) and not that this probably applies here.
It would be nice if Vrk1987 would return to provide more details. It is best if Vrk1987 does not sit on this for too long, as cancelling becomes more difficult the longer s/he waits.
There are a few ways other than being a minor where someone can cancel a car purchase contract. For a new car purchased from a dealer, for example, the terms of cancellation can be spelled out in one of the contract provisions. If the car (new or used) that is purchased is not as promised, the seller could be found to have breached the contract and the contract can be cancelled. And someone can walk away from a car purchase, but they risk losing their deposit and can be sued.
Thanks LdiJ, by the way. I think you are probably right.