The six year study followed the diets of more than 3,700 people 65 years or older.
The study found people with diets high in copper and saturated fat can add up to 19 years to their mental age.
The study also found that taking multivitamins resulted in people having higher copper levels.
Shirley Anne said, "I was really surprised because I thought vitamins were good for us to take, and I take a few, but I don't take a multiple vitamin. I take selected things like fish oil."
Copper is found in shellfish, nuts, potatoes, some fruits and chocolate.
The U.S. daily recommended copper intake is 0.9 milligrams. The study's subjects that consumed the most copper took in at least 1.6 milligrams.