As old as possible! I know in the USA, some young people are allowed to drive cars from the age of 14 years. In the UK, it is 17 years and on the continent of Europe it is 18 years. There is research to suggest that young people are FAR more likely to have accidents than older new drivers. If you have a teen who is wanting to learn to drive, then take them out as often as possible, get them as much experience as possible, both with you and with a registered, approved driving instructor. Continue to coach them after they have passed their test. Encourage them to consider their own attitudes to driving and don't let them drive their teenage friends around. Family members are ok and older friends (over 25). Teens in a car together is a recipe for disaster.