Pav, you can only purchase car insurance when you are 17 or over. However you can still get
car insurance quotes for 17 year olds by entering your date of birth as if you were already 17 on the application form. This will allow you to see how much you will end up paying for the car you are buying when you turn 17. Just remember that at the time of taking car insurance, you will then be entered in a formal contract with the insurance company and all information then and there will have to be accurate.
Just to reassure you, many people change personal details on their car insurance application to find out how much they can save before actually buying - be it car details like engine immobiliser, tracker, where the car is stored, pass plus certificate or date of birth.
If there's anything else I can help you with, do not hesitate to let me know.