While most people associate training with puppies, the reality is that dogs can learn at any age. Adult dogs are often easier to train than young puppies because they have more self-control. It's also important to keep training your dog as it matures.
House training takes time and effort in the short-term but gives you the long-term benefit of a dog who can be a part of your family. Realize that adult dogs adopted from shelters, rescues and kennels are often not house trained.
If you have adopted an adult dog, congratulations! You are one of the lucky people who will find out how rewarding it can be to give a good home to an senior dog. Adult dogs bond just as readily.