How old do the kittens need to be when you can hold them?

Well, their mother (if they have one) will probably not be too comfortable with you touching her babies. I would perhaps stroke them when they are newborn - a couple weeks, and then maybe hold them VERY carefully for short periods of time after that. The nwhen they start to play, you can interact with them, giving Mum a break. If they are orphans, handle them to feed them and cuddle them and give them plenty of love.