At the shelter where I volunteer, I think they already start giving solid food at 4 weeks. But I am not pretty sure, I hope someone else with more experience can answer this.

What I know is that they first start with wet food that they mix with milk to make a gruel. At the beginning they prefer the milk (especially if they still have their mother feeding them) so it can be hard to have them used to solid food. But the more you dilute the food with milk, the easiest the transition will be. And they use baby wet food, of course.