Milk can be given freely for individuals who are not sensitive or intolerant. If your pet is lactose intolerant you will see diarrhoea/ vomiting wherever you give him/ her milk. Allergies will show up with skin rashes and itching. If your pet is not showing any of these signs you can give milk freely.
Lactose intolerance being fairy common It's better to try milk only after 5 months of age, to make sure the kitten doesn't go through a tummy upset at a very young age.