How long until you can hold a newborn kitten?

Kittens, if the mother cat trusts you, can be held as soon as they are born. It depends if the mother cat trusts you enough to let you hold her babies. Holding the kittens, if the mother cat trusts you, is absolutely fine. A lot of breeders will hold the kittens every day in order to check their health and development, and to weigh them to make sure they are gaining enough weight. Some breeders will wear latex gloves as a precaution - to avoid any possible harmful bacteria transmitting to the kittens.

Another Answer:

Please note that I am not a vet! My cat had kittens and I was able to pick them up and hold them immediately after birth, but only as necessary. Example: to move them out of the way as she was birthing the others, or to make sure she did not lay on them. After the first day she allowed me and others to hold them, but only people she was familiar with and only in her line of sight. It all depends on the mother cat and her personality. Some cats will not allow anyone to be around their kittens, while others are more open. If your cat was very loving and liked to have you around before pregnancy, she is likely to have less of a problem letting you hold her kittens. Independent cats tend to me more protective. Also, if your cat is not used to a lot of people around her, DO NOT bring in people to hold her kittens. It will cause both her and the kittens extra stress, which is never good.

Another Answer:

My cat has just had her first litter during the night in the birthing box at the end of my bed. She didn't wake us up to have her kittens she did it all her self. 3 hours later she let us hold them without even trying to get them back. It just depends how caring/ protective of there kittens your cat is.