Puppies can leave at 8 weeks old. I will explain my belief on why this is the ideal time based on my educational discipline of animal behavior, and 23 years of training, competing, breeding, handling of dogs of all different breeds for all different working fields.

At the 49 days time frame, is that pups in a single litter can vary in developmental age by a week in each direction though all are born within a few hours. This developmental variation arises from several sources- conception can vary two to three days due to superfetation, delay in implantation of fertilized ova in the uterus may be another two to three days, location in the uterine horn, blood supply to the various embryos, developmental arrests or speedups, differential delay in parturition all can contribute to developmental variability. There is also differential post partum development especially during the first few weeks. This means that by the time the pup reaches 49 days since birth, it can be anywhere between 42 and 56 days old developmentally relative to all other pups in the total population of pups whelped on the same day, even to pups in the same litter. And it is the neural, physiological and physical development, not the exact chronological age, not minutes elapsed since popping into the world that is important in the behavioral stability or lack of it in pups and later when the pups reach adult status.

I put this finding first because I consider it the most important for putting the 49 days thing into perspective. Seven weeks is only a chronological age, only the number of days since parturition. Developmentally, it is an average of a large sample size with statistical limits of plus or minus a week. This would mean, predictably, 95 per cent of any population of domestic dogs at seven weeks after parturition will be between six and eight weeks old developmentally. Look at any litter closely and objectively each week for behavioral differences and you will see surprising variability. You will see some pups that are precocial, some delayed. What one pup does at a given age, others will only do next week or some did three days ago.