At least 8 weeks old, preferably 10 - 12 weeks.. Mum will probably try to kick kitties out at about 16 weeks anyway... (well my cat did, she just wanted to get back to her own life with no silly nonsense, feeding, licking, cleaning, fussing etc etc)
Definetely no earlier than 8 weeks. My sister's kitten was 8 weeks when she got him and he's always been a nervous clingy cat.The vet said he's probably been too young and that each kitten can vary greatly.