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