As a milk product I wouldn't feed it out of date to a neonate (i.e. a kitten under 10 weeks old). Above that age, I would make it up and offer it to them, on the basis their sense of smell is many times better than ours and they are very very good at detecting the aroma of 'off' food. If I am unsure whether something in my fridge is OK I offer it to the cats to taste, and if they reject it then it goes in the bin!