There's no minimum age for buying a car. If a billionaire gives his 5 year old child massive amounts of money, and that child decides to buy a Ferrari with it, there's nothing to prevent it. He can even drive it (if his feet can reach the pedals!) as long as he does so on private land.
However there's a minimum age of 17 years before a driving licence can become valid, and you can't take your theory and practical tests until it is. See here:
http://tinyurl.com/caq9ul
Chris