I think 18 is old enough to drink and to drive (not at the same time obviously).
At least with 18, they can go to jail if they kill or hurt someone when driving. You can't allow a child to drive if then you are not allowing to put him in jail if he kills someone.
16 years-old should be forbidden to drive alone. They should get allowed to drive ONLY if they go with a 21 years-old or older next to them.
In Andorra, they can drive that way. And not any adult can be by his side. Only a tutor or a parent that signed when the guy got his license. He should get another licence whe he becomes of age.
