Do they have their mom? If yes, they follow in much moms example, usually they will begin somewhere during this week... They nurse parallelly at least to week 8, often longer. Nursing being also partielly a sociale mom - kids thing.
If they are orphans, it goes a shade slower, but you can begin so slowely next week... And give them bottle at least two weeks more after they began to eating on their own.
When you change food to real food, make sure they do have access to litter, non clumping litter.
As their poo changes too, and mom doesnt no longer wants to take care of it.
At the same time, they get big enough to do it themselves...