Good breeders let their pups go at 10-12 weeks.
And thats when I am going to let my kittens go to.
Sorry to nitpick, but MANY outstanding breeders let puppies go at 8 or 9 weeks. In certain situations I would place a puppy with the right people on the 49th day after PAT testing.
Placing puppies at 8 weeks is VERY common in my breed, and is no reflection on the quality or lack thereof of the breeder.
There is nothing wrong with keeping puppies past that time if care and attention is paid to individual training, socialization, and attention. One of the best dogs I ever had I did not acquire until she was nearly 7 months of age. However, her breeder was careful to pay attention to her training and socialization needs.
In many puppies, 8 weeks CAN be a fear period. Placement can still go smoothly if care is taken with the puppy. Altho is is true that puppies can retain experiences at these ages for a lifetime, it is also true that puppies are very resiliant creatures as well who most of the time rebound from these experiences during fear periods, especially if they belong to caring owners who carefully socialize them.