Years ago, puppies would be weaned from their mother's and sold at 6 weeks of age. This is no longer acceptable practice. The breeder's code of ethics say a puppy should be 8 weeks of age, before being sold or adopted. There are 15 states which have laws governing the age by which a puppy may be sold or adopted to a new family and in most states it is 8 weeks of age. At Crooked Creek Cairn Terriers, puppies may go home at 8-9 weeks of age if being picked up, and may be shipped at 9-10 weeks of age.
Puppy mills and bad breeders will take puppies from their mothers at 5 weeks of age and sell them. Puppies don't even have all their teeth through their gums at 5 weeks of age. They've only been toddling around since 3 1/2- 4 weeks of age. At 5 weeks, they are toddlers in that they are starting to be playful but are clumsy and they still sleep most of the time. At 6 weeks, they are gaining coordination and are being weaned from their mothers, but most still nurse along with their solid foods. At Crooked Creek Cairn Terriers, we don't wean the puppies from their mothers. We allow the mothers and puppies to decide when to wean based on their instinct and individual needs. Puppies are introduced to solids at 4 1/2- 5 weeks of age, but most aren't eating on their own until closer to 6 weeks of age. Most of our puppies wean on their own by 7 weeks.
By watching puppies and their mother's instinct, you can tell developmentally when weaning and separation should occur. Mother's are so protective the first few days of life that they usually have to be forced to leave the puppies to go potty and eat/drink. It is their instinct to keep the puppies warm, and frequently nurse the puppies to nourish them and maintain the fragile puppies. Pupppies are like infants in that they have difficulty maintaining their temperatures initially. It's being snuggled up to Mom and littermates, that they stay warm. Their blood sugar stays stable by nursing frequently.
By the time the puppies are 1 week of age, the mother's instinct is still strong, but she is more willing to leave them for short periods of time. Watch out if you think you can touch or take them without her noticing though! By 3 weeks she's more relaxed about the human interaction and seems content to let you touch them and hold them. By 4 weeks she may leave the puppies for longer periods of time, just returning to them to nurse them. At 5-6 weeks, she's completely comfortable with the puppies being taken for longer periods of time, but is anxious to see them returned to her. She starts to wean them usually at 6 weeks, but some bitches vary in when they wean their puppies. By 8 weeks, the mother is annoyed by these little bundles of energy and says "See ya!" as the family takes their new puppy home.
My parents raised Siberian Huskies when I was a child and teenager, and as I said, it was common practice to sell the puppies at 6 weeks of age. Our Lobo would howl and cry for days when the puppies were taken from her so early. My father said he quit breeding because he couldn't bare to hear them cry for their puppies. But from what I understand now, if we had weaned them and sold the puppies at 8 weeks vs 6 weeks, their instinct would have been ready to give up the puppies and it wouldn't have been traumitizing.
So as you see, if we understand and listen to nature herself, we would understand why puppies should not be sold until developmentally ready. If a breeder sells puppies before 8 weeks of age, it may be reason for concern about the practices of that breeder.