There has long since been a great debate on the perfect or ideal age for a new puppy to leave its Mother and Littermates and be placed into their new homes. Public opinion and some studies have stated that the ideal age for a puppy to be rehomed is between 8-12 weeks.
When we actually examine Scientific Research Studies that have been done over the years, we find that there are many factors that weigh into determining the 'Ideal' age. Such as, what is the breed of the dog, does the breeder do socialization exercises with the puppies, does the breeder expose the puppies to different stimuli, are the new puppy parents experienced dog owners, etc.
Breeders are often scrutinized or called 'irresponsible breeders' for electing to send their puppies to be rehomed at 7 weeks of age. However, informed breeders realize the important benefits to rehoming puppies at 7 weeks. Here are some excerpts from a couple of articles:
'Puppies should go to their new homes between seven and nine weeks of age. This is the optimum time for a puppy to transition from his litter to his new family – any earlier and he may have issues with other dogs in the future; any longer with his littermates and he may not bond as well with people.' 
'The scientific data says that the optimal time to send a puppy to its new home is at seven weeks — 49 days to be precise. I never let a puppy go much earlier than that, regardless of a buyer pressuring me to let a puppy go at age six weeks, but I do try to get the puppies adopted as close to seven weeks as possible. Socially speaking, the litter is a competitive environment. If the puppies stay in the litter too long, they start to develop a pecking order and a strong pattern of dominant or submissive behaviors around their littermates. These carry over when they leave and can be a source of social and behavioral problems later on.' 
Its important to know what our dogs are 'telling' us. Learn to know what to watch for and how to 'listen' to our dogs in this short video of dogs interacting with each other and how they communicate their emotions and intentions.