A common misconception among dog owners is that the older a dog gets, the less likely he is to grasp the new knowledge you want to teach him. This is absolutely untrue about adult dog training. Although an adult dog that is set in his ways may process the info a bit slower, he can and will still learn. In fact, it does an older dog great good to keep processing new things.
Adult dog training (yes, even housebreaking) can be done with a dog as old as 14 or more! As long as the fella is not senile, he can learn. The way a dog grasps learning is simple. Imagine speaking English your whole life (knowing no other language but your own) and then being thrown into a classroom full of Spanish speaking students and a teacher who knows only her own language. You probably are not going to properly receive the info she is teaching until the teacher begins to show you what she means by training you to learn a special way. This applies to the ways in which all dogs learn, and adult dog training is no different just because the dog is older.
The best way to train an older dog is to always be very patient. As said, older dogs who are set in their ways may take a bit longer than a puppy that has no “ways,” but it will pay off in the end. Just take the time to show your dog just what you mean and teach him slowly and have patience with him. Adult dog training can be very rewarding, especially for a rescued pound dog if you only take the time to teach him to understand.