A minimum of 12 weeks!!! NO LESS THAN 12 WEEKS Many people think between 6 & 8 weeks but in between those few weeks the kittens immune system and many other body functions start improving or working on there own... The kitten didn't have to do anything but lay close to the mother and nurse! The queen (mother cat) even cleaned up after the infant much like human babies...
Kittens that leave their mother this young can become very sick and even die. Not to scare anyone but even though kittens can be VERY CUTE at 6 weeks don't take one home that young! You will be much happier to have a healthy silly kitten that's a little older, than a sick, helpless newborn...