As I commented, I think this is more of a developmental milestone issue as opposed to specific ages but there is a reasonable rule of thumb. As with all of us, maturation may happen at different rates and it's really more important for you to know when a given puppy is ready for adoption.
So, some milestones periods to consider:
- Separation of a puppy from litter mates and mother before or at 6 weeks of age can lead to recidivism in behaviour (Elliot and Scott, 1961) and tend to show negative effects on health and weight (Slabbert and Rasa, 1993).
- Separation from mother at time of weaning can lead to significantly increased vocalization behaviours (Elliot and Scott, 1961).
- Stress during the house training phase can lead to poor learning behavior. Puppies start forming preferences for elimination by 8.5 weeks (Fuller, 1967) and up until they do, they tend to just go anywhere as they need to go.
- Leash training is best between 5 - 9 weeks (Scott and Fuller, 1965)
General source: Manual of Clinical and Behavioral Medicine for Dogs and Cats by Karen L. Overall.
What that means, and the book goes into a lot more detail, is that there is some variation by dog, but that 8 weeks is really a minimum for a puppy and a bit longer is probably better. A good breeder should take the proper time to socialize, house train, and accustom the puppy to leashes and other restraint objects and if done for at least 8-9 weeks (or longer, which is better) the dog will be much better suited for adoption and have far less issues for their human companions in the future.