I had always thought kitten food was up to 1 year, and thereafter they went on to adult food. I do agree that so long as they are fed a good complete food, it shouldn't really matter. I can certainly sympathise with the difficulty of feeding adult food to one cat and kitten to another - the nutrition may not be sugnificantly different, but our adult Riley certainly prefers Skye's kitten food!
I would suggest that, unless its a slow growing breed, or you feel that they are still on the small side, or are slow to bulk up, keep them on kitten food a bit longer, but otherwise you can probably start switching around 10 months. We had Riley on 1/2 kitten, 1/2 adult from 10 - 12 months, then had him on adult only, and he was fine.