When I was in Kindergarten we got our first cat. Every year she would have about two litters of kittens. As you might have guessed I was raised around a lot of kittens. In my opinion, kittens can be separated from their mother at eight weeks. For outside cats that is long enough for the kittens to learn to take care of themselves. For inside cats it is long enough for the kittens to have been liter box trained and to be eating solid food.
They can be separated from their litter mates as soon as they can be separated from their mother. When the kittens are separated, according to their personality, they can adapt or in rare cases will become depressed. If the kitten is depressed I suggest getting another cat for it to play with.
Eight weeks is the perfect time to separate the litter mates because that is the time when they are most rambunctious. Since they are so rowdy at this time of their life most kittens don't even know that they have been separated from their mothers or litter mates. This loses the emotionally strain that separation would have on the kitten. The eight week period gives the kitten time to stop nursing. If the kitten is still nursing, separating from its mother will not lead to it not eating. The cat will just learn to eat normal food.
If the mother is gone but the litter mates are still with the kitten then the kittens will become more active. With "mommy" watching over them, they stay in close proximity and don't want to leave her side in case of a meal. When the mother leaves the kittens they are intent on more things than just food. The kittens become more curious about many things and can be a big mess.
When separating the kittens form their mother and litter mates you must know the kittens personality to find out how they will react to the separation, but I recommend not separating them until the kittens are eight weeks old.
If you do separate kittens before the eight week period, the consequences may be severe. The kittens could be lost without their litter mates. They would turn into savage kitties and be totally demonic. The kitten could also contract a cold or flu from not having the nutrients that their mother's milk provides. If the kitten has not been weened it might die of starvation because there is nothing for it to eat. It might also die from choking on a piece of hard food it tried to eat. While separating them too early is dangerous so is separating them too late.
It you wait too long to separate the mother cat and kittens they could become attached to each other or start inbreeding. When you finally do separate the cats they could become depressed because of the lack of company. If you do not separate to mother cat with her kittens and then you don't have them spayed the kittens could breed with each other and/or the mother.
It is a good idea to separate kittens with their mother and litter mates at eight weeks old for emotional, physical, and psychological reasons.