willdwood is correct, I puppy walk Guide dogs, and we start them at 6 weeks. They have a small area outside, preferably paved, that they are allowed to use as a toilet. When the pup has performed they are praised with the words "be busy" for a wee and "big busy" for the other. Within weeks they are programmed to go on demand. This is done before the dog leaves the house so avoiding any accidents. The dog will eventually go on the same paving slab. All but the stupidest dog can easily learn this. They seem to automatically know not to defacate in harness, probably because they have to concentrate so hard when working there is no time for sniffing where other dogs have been. Any dog who was unreliable with this requirement would not be considered as a working dog .