How long do I need to feed my kitten special kitten food?

I’ve been feeding her Kitten Chow, but it’s causing issues for my older cat. When Kitten Chow is around, the adult cat won’t touch her regular food. Apparently, Kitten Chow is infinitely yummier. So, I finally gave up and have been feeding both of them the kitten food. Poor older kitty is getting a bit fat, though, and it’s starting to impact her agility. I’ve already tried doing a mix of adult and kitten food, but kitten lost weight on that plan. At this point, I figure I’ll just keep on with the kitten food as long as the kitten needs it, and deal with the older cat’s weight at the end of that period.

BOTTOM LINE: The Kitten Chow package says to use it for a year… is that really necessary?