It is illegal to smoke in a vehicle if there is any other person under the age of 18 present in that vehicle. This means that while there is more than one person in a vehicle and one of them is a person under the age of 18:
Therefore, if both the driver and the passenger smoke while there is a person under 18 present in the vehicle, the driver can be given 2 penalties (once for smoking and once for not preventing another from smoking) and the passenger one (for smoking).
This ban on smoking doesn't apply where the driver is alone in the car, even if the driver is only 17 years old. The ban on smoking also doesn't apply to a convertible vehicle when its roof is completely down. E-cigarettes are not included in the banning regulations.