It is recommended that your dog is not younger than six months, but it all depends on the breed. Somewhere between six months and a year is the right time to start training most dog breeds, so consequently, this is the right time to get them used to an invisible fence system as well. Still, it is advisable to consult your veterinarian and together come up with the right time to start training your dog. If it's a stubborn one, you can start earlier to get them used to the entire system. But if your dog is already apprehensive and scared, don't push it.