After working with a client the other day their neighbour pulled me over and asked if there was any hope for a dog of 'that age'. I pointed out that as the dog was only 18 months old there was nothing to stop the learning process except the owners themselves.
He was surprised to discover that dogs, like humans, can learn throughout their lifetime and benefit from training from puppyhood to the grave. Yes, like humans there are particular times in a dog's life when he's much more sponge-like but as they age they still enjoy new challenges. So whatever age your dog is, give them stimulating things to do and keep working with them - obedience, agility, trick training, flyball, the options are so plentiful and so varied neither of you could possibly get bored!