I think they should learn while they are growing (It's harder to learn as you get older) so that they leave home, they will be a pretty good, confidence driver.
plus, teens have jobs and sometime babies, so they need their license to take responsibilities.
So 15 or 16 is a good age.
I learned to drive when I was 21 years old, and I am still a nervous driver. I plan letting my son get his learners at 15 and if he shows he can make good grades and take responsibility to do his homework, he can get his license at 16.. if he keep making D's and F's and show irresponsibility in schools, I'll keep him in his learners until he is 18 years old.
If he make straight A's, then he can have his own car but if he quit making good grades, I'll have to sell the car and let him borrow mine whenever he need it,... Unless if he is failing after doing good, Then he can't drive independently at all...