Here in the Netherlands it is illegal to seperate kittens from their mothers before they are seven weeks old, breeders are obligated to keep their kittens for 13 weeks (and minimum weight of 1 kilogram) and cat behaviourists often advise 16 weeks because that is the most natural state of things.
It depends on the situation the kitten is in of course, a wild kitten in a rescue situation might benefit more from going on to a new home earlier because there it could get more socialization than on the street, or a kitten without mom has nothing to lose anymore, but in an ideal situation a kitten will continue learning from mom as long as it stays with her and try as we might, a kitten can never learn all things it needs to know the same way from us as it would fron a real cat mom.
If the mom is still present and the kitten is in a safe environment, it is not only socially better for the kitten to remain there for longer, but also physically. A kitten needs time to develop its immunesystem. Kittens that are 7 weeks of age do not have a fully developed immunesystem, are not completely vaccinated yet and still benefit from antibodies in the mother's milk. To take away mom is to take away that protection too.
If you adopt a kitten from a family you can always ask them to vaccinate the kitten there if you pay for it.
I am sure a lot of people will chime in to say that with just a little love the teeny tiny kittens will be OK too, and that they adopted tiny kittens and they are all fine. It would be nice if that was always the case (or if more people were better at realizing that their cat does actually have some behavioural issues, they often go unrecognized). Ernesto and Mimosa were born on a farm and came to live with us when they were 7 weeks old, it wasnt until we got two purebred kittens that were placed at 3 and 4 months old that we realized Ernesto and Mimosa acted differently and had some problems.