Whenever I had litters of my own I can tell you from experience that I would never let just anyone handle my babies without making sure that they passed several guidelines first.
Things like - was the person who wanted to visit the puppies: in bad health; had they been around another person, animal or pet that was sick; had they visited another kennel or vet clinic before they came to my house; etc.
I am very picky with my German Shepherd babies. If the answers to any of those questions was yes then I did not let that person come over to visit my babies - and even at that, we had no scheduled visitors until after the babies had had at least their first set of vaccinations and wormings.
It's easy enough to bring things (diseases) over that you don't know about so I at least try to monitor the ones that I can. I try to protect my GSD babies from everything that I can.
But as far as handling goes, the whole adult family (and older kids) here helps to handle the puppies and help momma with the newborns throughout the whole growing process.
I like to handle them frequently and on a daily basis to get them used to having their feet touched, to being picked up, to being on their backs, etc and to be talked to softly as they mature so that they are well used to human contact the whole time.
The more handling a puppy receives as it matures, the better it is for the puppy. This is the job of the breeder and their family but as far as strangers handling puppies goes (without family consent), I would expect 6-7 weeks of age from most breeders would be about average.
