How come?Is it the same as with dogs,as in socialisation,learning manners etc etc. Most kitts i know of go at 8 weeks.Kittens don't mature anywhere near as fast as puppies. They suckle their mothers for a minimum of 10 weeks, often right up to the day they leave at 13 weeks. They don't start on solids until later -puppies 3 weeks, kittens usually 5-6 weeks, I have had the rare litter that started at 4 weeks (each time ONLY because the mother had run out of milk and so they were encouraged to start, and each time it gave the kittens gastric problems as their bodies weren't ready for solids yet) but it is also very common for kittens not to touch solids until 8 weeks. I have had 2 or 3 litters where one kitten didn't touch any solid food until 10 weeks old. It's slightly different in moggies because such litters are virtually never well reared, and so the kittens are forced to start earlier, the mother won't have enough milk etc etc, or the kitten is sold/given away and have no choice, but it isn't good for them and the best start for any kitten is mother's milk alone for at least 5-6 weeks, then solid food AND mother's milk until 10 weeks minimum.
Every time I have a litter and look at them aged 8 weeks and see just how often they suckle their mum and how much they still depend on her I am horrified at the thought of all those poor kittens taken away at that age -it's very cruel indeed. (Letting kittens go at 8 weeks could be compared to letting pups go at 4-5 weeks.) I've had rescue moggy kittens at young ages and they don't get a chance to develop normally at a more natural pace -and they often end up with behavioural problems for life, even if some are things that people don't always notice as they think it is normal for all cats.(Such as excessive kneeding, and also drooling. I had one rescued mog that arrived at 8 weeks and desperately tried to suckle my hair looking for milk, he never stopped that habit during all his life.) Given her own choice, a queen will not stop suckling her kittens and encourage them to leave/be independent as early as 8 weeks, it's more like 3-4 months.
Socialisation of course isn't the same as in dogs either as you don't have to get kittens used to different situations, and anybody who rears a litter well will spend time with the kittens so they get used to people -it doesn't even have to be a lot of different people. Not to mention that having a friendly, outgoing mother AND father is the very best way to get an outgoing kitten.
Cat fancy rules also say you must have had kittens fully vaccinated before they are sold, and that means first vacc at 9 weeks, second at 12 weeks, and one full week left before they are sold. But even if this wasn't so, at 3-4 months of age kittens are just right to leave their mothers and can cope far better than at earlier ages. This is when they start to become independent and want to start exploring their surroundings on their own.