I've seen it said over and over on the board - that puppies need to stay with their mom and siblings until 7-8 weeks of age - but I don't think anyone has ever really explained why. I thought it might be helpful for those people looking to purchase a puppy to know why it's so important.
Puppies can start eating solid foods between 3-4 weeks of age. Some pups can be weaned (stop nursing on mom) as early as 5-6 weeks of age.
Just because a puppy is eating solid food and not nursing does NOT mean they should be removed from the bitch.
The bitch is not just a milk factory and cleaning lady. She will have a VERY great influence on the puppies early development. Her actions and reactions will help (or hinder) in shaping the puppies temperament.
For example, let's say I threw a metal pan into the whelping area so that it makes a very loud noise. If the bitch runs and hides from the noise in fear the pups will see this and most will react the same way. The puppies think "If MOM thinks it's scary then so do we".
But if the bitch either doesn't react or approaches the pan, the pups will see this and react the same way.
A dogs' temperament is influenced by nature (the genetic material used to create the dog) and nurture (the way the dog was raised as a puppy).
The bitch (at least a GOOD bitch) will also maintain order in the puppy pack. A bully puppy that constantly picks on the others can find themselves being reprimanded by mom. A puppy that plays too rough with a sibling might just get a "time out" from mom.
These actions by mom help puppies learn what is acceptable and what is not and they learn how to behave around other dogs (both puppies and adults).
Puppies need to be with their siblings to learn about dog body language, dog social skills and bite inhibition.
When puppies play with each other and one gets too bitey, the other pup will cry out and move away. After a couple of these encounters the offender learns that if they bite too hard the fun stops. That's natural bit inhibition.
Puppies learn the same thing when they interact with their mom - bite too hard and you get in trouble!