If you are ready to adopt a kitten as a pet. Do really consider adopting in a pair if given a choice. To separate a kitten from the mother cat, it should be about 2 months old, when the kittens are able to eat on their own. Separating it from the mother cat is a necessary step as this is the only way the kitten is going to a new home, but leaving their little playmates behind can be an emotionally hurtful experience.
Kittens need to socialise with their own kind. Having their playmates around allows them to be occupied with playing. You can see kittens like to bite, wrestle with one another and chase each other around. As humans, we are not able to indulge in such activities with them in the same way that fellow kittens can.