According to the American Society for the Prevention of Cruelty to Animals, kittens should start nibbling on dry food at about five to six weeks of age. They should begin dry food even if they are still nursing.
Orphaned kittens who are not able to nurse from their mothers can start eating moistened kitten food at about three weeks of age. Mother cats tend to wean their kittens by about eight weeks, at which point 80 to 90 percent of the kittens' food should be dry food. Kittens should eat food that is especially designed for kittens rather than adult cat food for the first year of their life because kittens require more protein than adult cats.