There really is no time that is too early to wean dogs - sometimes the mother dies or is unable to provide the puppies with the necessary milk, so they have to be hand fed either by syringe or bottle. The problem is they really need to have some of the colustrum, which is the very first milk the mother produces after the puppies are born. This gives them immunity to diseases and helps them to develop properly. There are artificial ways of giving them this and there are lots of different types of powdered milk that is made especially for orphan or weak animals.
If you mean weaning the pups onto solid food, this is usually started at about 4/5/6 weeks, although it can be done as young as two weeks under exceptional circumstances, such as if the mother has not got enough milk or there are too many puppies for her to feed. They are taught to lap milky or softened complete food, or raw mince, and from then they learn to eat 'proper' food.