kitten food is rich in things that your kitten might need for growth and health. most of the adult foods might be lacking in these things in the amounts that the kitten food would have. also a lot of adult foods are based on the average 'neutered' cat or cat that has gone through hormonal changes already. They say a year old just as an average. I personally feed my kittens kitten food until they are 9 mths old unless they are under sized or had some complications as kittens. it's a personal preference i think.