Age is Important
The most impressionable period in a dog's life is its first 12 weeks. Weaning takes place at six weeks, but there is still much for the puppy to learn from its mother and littermates.
Puppies should stay with their mothers until eight weeks old - a little less for puppies from large litters and a little more for puppies from small litters.
Around eight weeks is the ideal time for a puppy to move in with its new family. It certainly should do so before 11 or 12 weeks of age, to ensure that it is most receptive and relaxed in its new environment.
Some breeders keep older puppies in their kennels, planning to show them; however, dogs raised in kennels do not always make good family dogs.
If you are getting an older puppy or adult dog, make sure it is house-broken and used to living in a home with a family. A mature dog is more likely to suffer separation anxiety or have other behavior problems.