They divided the volunteers into five groups based on how often they ate green, leafy vegetables including spinach, lettuce and kale.
Nevertheless, Dr Sara Imarisio, the charity's head of research, says in an emailed statement: "Fruits and vegetables are a key component of a nutritionally balanced diet, but figures suggest that many of us struggle to eat our 5-a-day".
Researchers from Rush University and Tufts University studied almost 1,000 people and found out that those who ate one to two daily servings of green vegetables like lettuce, kale or spinach had a significantly lower rate of cognitive decline. The people in the top serving group ate an average of about 1.3 servings per day.
As many as 960 elderly people - 81 years of age on average - took part in the study, in which they were offered to fill in a questionnaire, indicating what they usually eat.
The study was published today in the online issue of the medical journal of the American Academy of Neurology. However, signs of ageing of the brain manifested themselves to a lesser degree with those who ate a lot of green vegetables every day.
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"Projections show sharp increases in the percentage of people with dementia as the oldest age groups continue to grow in number, so effective strategies to prevent dementia are critical".
Those eating the most green veg ate an average of around 1.3 servings a day - those in the lowest group ate an average of 0.1 servings.
Overall their performance on the thinking and memory tests declined over time at a rate of 0.08 standardised units per year.
The scientists said this was equivalent to being 11 years younger in age. The study conducted by a team of researchers at Rush University Medical Center in Chicago claims that regular consumption of green vegetables can improve brain health.