Answers 0 On average, the earliest a kitten should be considered an Adult cat is at 1 year old. For dietary purposes, kittens can start eating adult cat food, anywhere from 9-12 months. At 7 months, a kitten should have grown all it's cat teeth in.. On average, the earliest a kitten should be considered an Adult cat is at 1 year old. For dietary purposes, kittens can start eating adult cat food, anywhere from 9-12 months. At 7 months, a kitten should have grown all it's cat teeth in.