My youngest kitten decided she would rather have adult food than her kitten food when she was about 8 weeks old. And believe me she is still growing without any problems.

I have seen this come up on here before, I seem to remeber some breeders on here saying that this was not a problem, so I didn't worry about it.

I think it is advised to start the change over from about 6 months if you are trying to do things by the book. Just out of interest, what brand of food do you feed?