I think it would be fine to feed your kittens adult food but be sure to mix it together when you start and mix it together for at least a week if not a little longer. You could start with 3/4 kitten food and 1/4 adult cat food. And after 4 or 5 days, do 1/2 and 1/2 and then move to 3/4 adult, and 1/4 kitten. I had some feral kittens that couldn't eat the kitten food and so they went right on adult food at the age of 4 months, so yours should be just fine.

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