No earlier than two years. Think of it as a human having a child, a girl can get pregnant at 12 but should they have a baby that age? NO. Plus, why would you want to breed your dog when there are millions of dogs in shelters getting euthanized because they can't find a home. Think about it, this world doesn't need any more puppies.
It can happen at six months. It shouldn't because they are too young still to carry pupppies but it can happen.
As early as 9 months.