On average, the earliest a kitten should be considered an Adult cat is at 1 year old. For dietary purposes, kittens can start eating adult cat food, anywhere from 9-12 months. At 7 months, a kitten should have grown all it's cat teeth in.
Technically speaking, vets usually conclude that cats are Adult cats at 1 years old. So once a cat turns 1 yr you should switch from kitten food to regular adult cat food.
For comparison and reference, The Cat Bible, a book by Tracie Hotchner, provides the following Cat years to Human years comparison list:
1-month-old kitten = 6-month-old human baby 3-month-old kitten = 4-year-old child 6-month-old kitten = 10 human years old 8-month-old kitten = 15-year-old human A 1-year-old cat has reached adulthood, the equivalent of 18 human years 2 human years = 24 cat years 4 human years = 35 cat years 6 human years = 42 cat years 8 human years = 50 cat years 10 human years = 60 cat years 12 human years = 70 cat years 14 human years = 80 cat years 16 human years = 84 cat years