6-MONTH-OLD KITTEN EATING WEIGHT CONTROL FOOD FOR ADULT CAT

Good morning. I was wondering...I have a kitten that's 6 months old
and a 10 year old cat. The older cat has been eating the higher
calorie food for the kitten. Our local vet said it's o.k. to give
both cats the same food..the one that the older cat eats..Iams weight/
hairball control...the cat is small and always seems hungry..am I
making a mistake in not feeding kitten food to her until she's 1 year
old?..or can I just keep giving her larger amounts of the "diet"
food. I'm suspecting I may be depriving the kitten of the fat and
other nutrients she needs for proper growth. Any and all help/
suggestions are greatly appreciated..Joe (Danbury, CT)