Via the New York Times, scientists have been able to extend the cognative abilities of older dogs through a combination of enriched diet and enriched environments.  Per the study:

By the end of the two-year trial, it was clear that the enriched diet alone and the enriched environment alone were each helpful in preventing decline. But the mental functioning of the dogs given a combination of enriched diet and stimulating environment was considerably higher than that of the dogs in the other three groups, the researchers found.

This study was conducted on beagles, already an intelligent breed (like most hunting types).