This as much depends on the mother as it does their age. As with dogs, your relationship with her is very important. If she is comfortable for you to be in the vicinity of the nest you can safely try to stroke the kittens. Under normal circumstances you should avoid too much handling but just enough to make them used to human contact from as young an age as possible.

Never move them out of sight of the mother and if she behaves abnormally or over-anxious try again in another few days. To begin with, you may like to hold one for her to lick (as if you are assisting her with her duties!) and it may also be useful to help them arrange themselves at milking or, when they are slightly older, to stop them from being so aggressive with their mother!