Varying dogs and breeds mature at different rates. Young puppies are not yet ready in terms of their cognitive development. They can easily become distracted to understand and be able to absorb the training in setting boundaries. One way to know if your dog is already mature is that they should be able to follow basic commands such as sitting, staying or come when called.
Different brands have different age requirements. Some can start as young as 8 months, as long as they are already able to follow instructions.
Dogs that are pregnant are not also recommended to use this wireless fence system. It is best to wait until they have given birth or they are able to recover from delivery