Kittens up to 12 weeks can digest milk from their mothers. This milk contains far less lactose (milk sugar) than cow’s milk.
After 12 weeks, most cats lose the ability to digest the lactose in milk. If your cat is suffering from lactose intolerance, it will show symptoms of diarrhea or flatulence.
Adult cats do not need milk as a nutrient, so it’s best not to provide it.