Are you deaf, dog?
The hypothesis that dogs grow more deaf as they grow older, has indeed been proven to be true.
A dog’s extremely sharp hearing and sense of smell makes it very suitable for hunting and to serve as a guard dogs. Their hearing enables them to detect the faintest of sounds. Police K-9 units deploy dogs, usually German Shepherds, to help solve crimes. Police dogs also help sniff out contraband, drugs and other dangerous substances.
To improve the reliability of our results, we could have perhaps used a larger number of breeds or tested dogs of a wider age gap. We could consider repeating this science fair project by comparing the difference in hearing deterioration between small breed dogs and larger breeds.
Another possibility would be to compare the effect of age and hearing in other domesticated animals like cats and birds, and perhaps even fish!
Hearing range - http://en.wikipedia.org/wiki/Hearing_range
Aging in dogs - http://en.wikipedia.org/wiki/Aging_in_dogs
Ensure that the dogs are handled at all times by a qualified dog trainer, to avoid being bitten/injury. It is possible that the dogs may become agitated by the high frequency sounds.