Is it harder to trainer an older dog than a puppy?
Puppies are certainly quicker to learn than adult dogs, but it is possible, with patience, to teach an older individual successfully. You need to bear in mind that mature male dogs especially are likely to be more assertive, which can create problems; bitches and neutered dogs of both sexes are likely to respond more readily to training.

It also depends partly as to whether the dog has received any training beforehand. Clearly if it has, then you can build on this base. A similar routine as recommended for puppies should suffice, whether training an older dog to be clean in the home or obedient.