The longer the pup stays with moms, the better. My pups stay with me till 11-12 weeks...but I've kept a couple longer 13-14 weeks.
Now, I've purchased one male (unknown to me since I didn't leave with registration papers and breeder lied about age) that was a little over 6 weeks old. Once I received the papers...months later, is when I realized his true age.
My most recent male was a little over 8 weeks when I got him. I would not suggest getting a puppy younger than 8 weeks. Like I said, the longer the breeder keeps them, the better for the pup. They learn lots of socialization skills from their mom and litter mates during this time.