A method for inhibiting a decline in learning and/or memory in an animal comprising maintaining the animal on an antioxidant-fortified diet for a period of at least about 3 years. The diet comprises at least one antioxidant in a total antioxidant amount effective to achieve such inhibition. Suitable antioxidants include vitamin E, vitamin C, .alpha.-lipoic acid and antioxidant-containing plant meals. The methods are particularly useful for young adult canines such as dogs.