A person under the age of 18 can actually get credit for any item that is deemed as a necessity.
As an example, a car could be seen as a requirement if a 17 year old needed it for work as they lived in the middle of the countryside with no public transport links.
The case law actually goes back many many years and involves a young man purchasing a number of waistcoats via a loan.
That said, finance companies are very reluctant to lend money to under 18s be it for a car or any other purchase even if a guarantor is available.