The development of puppies shows that the majority of their personality/ability to handle situations is developed between the ages of three and 12 weeks.
Many people believe there's a fear period that runs from 8-11 weeks old or so. This is why a lot of breeders opt to send their pups out right about the eight week mark so that it hits (hopefully) right before the fear period. Younger than that is not ideal as the pups truly need the interaction with their littermates for proper social development.
A breeder determines the personality of their pups nearly 100%. Not only are they genetically responsible, they have control of those pups for the early part of the social development (and usually for the majority of it). I went to a seminar with John Rogerson (an English behaviorist who does puppy training) and he straight out said that breeders are completely responsible for the temperament of the puppies they produce. In the period from three weeks to eight weeks, puppies should be introduced to numerous surfaces, sounds, interactions, etc. The breeder is responsible for this, and for making sure that the new owners continue this. A breeder who just lets the puppies live out in a kennel - or even in their home - and makes no effort to daily change the environment and expand the experiences the puppies have is a poor breeder.
By the time the pups are eight weeks old, they should have myriad experiences to fall back on. They should be comfortable with a collar and leash and a crate. They should be comfortable riding in a vehicle. They should have heard and learned to feel safe through dozens of sounds. Breeders are molding these little personalities into the dogs they will be.
I don't have a problem with a puppy leaving the litter at eight weeks as long as the breeder has done things properly and the new owner has been well-informed as to their further responsibilities (and agreed to follow through). It doesn't do a puppy much good to stay longer with a breeder who isn't doing the right things. I prefer to have a new pup come to me at eight weeks old because there are LOTS of things I'd like to continue to imprint the puppy with - things that may be more unique to my situation that the breeder can't simulate.
Done right, an eight week old pup should be confident and outgoing. I don't accept breeder's excuses that "this breed is just shy and scared at this age" (said by a prolific chow breeder not long ago). If you're breeding good temperaments and you're doing the socialization and handling that SHOULD be done prior to eight weeks, by that age the puppy should be able to tackle the world! (so to speak .. *L*).
I have to say that Rogerson's idea of a puppy kindergarten class was fascinating to me (and I'd implement it if we had enough puppies in this area). He accepted six puppies/familes at a time. The first night they met at his home .. everyone brought their puppies and treats, and all the treats were put together on a table so everyone shared. They met seven nights in a row - and each night met at another person's house. So by the time the week was up, the puppies had been to six new places, met dozens of people (families with kids and all that), met other dogs, heard lots of new noises, etc. - and they were highly socialized and ready to deal with most things! And this had to be done before the pup was 12 weeks old. I just found this to be such a neat way of doing things - and thought it made much more sense than a weekly puppy class that always met in the same place and that couldn't truly take good advantage of that small range of time before 12 weeks old.
I know, people will think "what about the vaccinations??" .. well, from a trainer's standpoint, the risk of having a vaccinated eight week old puppy catch parvo in a group of vaccinated puppies is much lower than the risk of having a dog with aggressive or fearful behaviors for the rest of its life because it didn't get the proper socialization and early training it needed. We all have to balance our risks ..
Melanie and the gang in Alaska