Puppies should be separated for sleep, napping, and for separate play times with you by 12 weeks of age. If they are not separated for periods of time at about this age, they can form too strong of a bond with each other, and may be less likely to form strong bonds with humans.
Separation from littermates is an important part of the growing up process for puppies.
I begin separating my puppies into pairs or 3s in crates for sleeping and nap times at about 6 weeks. At 8 weeks, they get their own crates for sleeping and napping. ( That's right....My puppies go to their new homes crate trained....AND leash trained.
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The puppies I keep are rotated so they have different playmates each day.