The proverb is often used to mean that older people (or animals) can't learn anything new. But it's not really true, either in the human or the canine world. You can teach old dogs plenty of things. In fact, almost any command, skill or trick that you can teach a puppy, you can teach an older dog. It might just take a little longer. Researchers at the University of Veterinary Medicine-Vienna worked with 145 border collies ranging in age from 6 months to 14 years old to see how attention and concentration changed with age. First they did two tests to see how quickly dogs responded to objects and humans. The older dogs tended to lose interest in the object alone before the younger dogs did, but if there was a person with the object, the older dogs were as interested as the younger ones [source: Science 2.0]
Your mature pooch may be only too happy to have you believe it's useless for him to learn new things. Especially if there's a nice, soft bed in a warm patch of sun waiting for him. But just as aging humans can keep their minds engaged by doing crossword puzzles and the like, dogs are best served when they're continually challenged mentally. So grab a bag of dog treats and get to work.