In immature dogs (less than 8 months of age in small or medium dog breeds, older in larger breeds), damage to joint cartilage can occur. (vin.com)
They want to see the results of each mating as soon as possible, which makes them use young and still immature dogs in the breeding. (breedingbusiness.com)
They also have a tendency to expect immature dogs (puppies) to behave like adult dogs. (samthedogtrainer.com)