Whatever age you settle on, driving lessons are a must. And set limits for the first few years until they get some experience.
Rules by us are, no night time driving out of city limits, no driving past 1:00am and no answering cell phones while in motion (texting is an obvious no). And if you were not in bed by midnight, you cannot have the car the next day.
And no more than 4 people in the car at one time.