My vet said cats should come off kitten food at 8 months, but she's one of those "GP" type vets, knows a little about everything and can answer general questions.
All the cats we had growing up were always skinny looking until they were around 10 months.
Here you go, website sited at the end:
Thin Cat
Usually you can tell when a cat is too thin. Its fur may be dull, and it probably has an unhealthy look. According to the weight chart, you may also see the following:
The skeletal structure is easy to see. Ribs, vertebrae, and pelvic bones can be seen and felt.
The neck is thin.
The cat's waist is unhealthily narrow.
The abdomen is concave.
There is no obvious fat.
Underweight
A cat may be underweight without being unhealthily thin. There are many reasons that a cat could be underweight, including having worms. If your pet is underweight, you may notice the following:
While the ribs might not be easily seen due to the fur, they will be easy to feel.
The backbone may be easy to see and will be obvious when you run your hand over the cat's back.
There will be little fat on the cat's body.
The abdomen may be concave.
The Ideal Weight
A cat that is the ideal weight will be healthy and have a longer, higher quality of life than a cat that is too thin or too fat.
The ribs can be felt, but they will not be visible when you look at the cat.
Looking down on the cat you can see a waist.
The abdomen will be slightly concave; this creates a smooth curve from the chest.
Cat Weight Chart | LoveToKnow