I have heard a lot of excuses for weaning too early. Ask for, five, and six weeks of age. It is not too hard on them. A puppy is simply not ready to leave mom and siblings at 4 to 7 weeks and in most cases not at 8 weeks old either. If the mother is taking care of medically, nutritionally, and emotionally it is not hard on them.
Mother knows best.
She will wean them when it is time, and the puppies will be healthier and more emotionally stable. And that is what should be strived for. Along the same lines of emotional stability and health you want to ask this very important question. Our answer is always that it depends on Mom and the individual puppy. If a breeder says seven or eight weeks, find another breeder.