Imagine being in a group of friends and telling your older dog "Wag if you like girl dogs" or "Wag if you're obsessed with squirrels". Having a dog that knows how to wag on cue can be very funny. You can come up with a lot of jokes that involve wagging to entertain friends. You can also substitute the cue word "Wag" for another hilarious word or phrase by using the same training methods. Perhaps you want your dog to wag every time you say "Cat", you can simply replace the word "Wag" in your training practices with the word "Cat".
Because your dog is older it can be hard to find new tricks that are not too physically demanding. Many older dogs still enjoy learning new things and still require mental stimulation to be happy. Training can also be a great way to bond with your older dog. Because 'wag' is not a physically demanding trick, it can be the perfect trick to teach him. Plus it can be lots of fun.
This command is relatively easy to teach. The most important thing to remember is to not take yourself too seriously. To train this command, you will need to make your dog feel happy and excited so that he will wag his tail. Do not be afraid to act silly. This is the time to break out the high pitched voice, to dance a bit, to smile and laugh at yourself. Make things fun and enjoy yourself. If you are feeling frustrated because your dog will not 'wag', definitely take a break. You will get no closer to getting him to wag his tail if you are not feeling happy yourself. Wait to try again when you are feeling happier and perhaps even a bit goofy.
Expect this command to take between two and six weeks to train. Some dogs will naturally catch on more quickly and learn this rather soon, but others may struggle with the transition from wagging because you made them happy to wagging in response to the command. Be patient with your dog and if he is struggling, keep practicing the trick with your funny voice or toy or treat for longer. You will need to practice with the fun things included for long enough for your dog to learn to love the word "wag" so that he will wag when he hears it.
This command may never work if you give it to your dog while you are in a bad mood. Even when your dog understands that he is supposed to wag when told, if he feels sad or scared he will not be likely to do it. It's like someone angry telling you to laugh or smile. That would probably be the last thing that you would feel like doing. If someone were to tell you "Smile! Smile!" in their silliest voice though, you would probably easily smile.
To get started you will need small tasty treat if you are using either the 'Treat Luring' or ' 'Happy Voice' method. If you are using the 'Toy' method, you will need one or more of your dog's favorite toys. If you are using the 'Happy Voice' method, you will need your most excited, silly, and happy voice. You will need some level of a happy voice for all of the methods, though. The more excited, happier, and goofier the better. Prepare to get into the training spirit at least as much as your dog. Perhaps you will even have more fun than he does. Finally, you will need patience and a great attitude to help your older dog learn.