There is no methodological merit to this claim, just a hunch from a public health specialist and farmer.
First, some science... Our brains develop through various activities and as a result of various factors, such as nutrition. A large part of brain development happens in utero and during early childhood, but it doesn't stop as we age. What we do as older children, teenagers and adults continues to play a role in the development of our brain. Alzheimer's disease is the most common type of dementia, a condition of the adult brain marked by damaged brain cells or damaged connections between brain cells. Recent statistics indicate that 1 in 8 adults over the age of 65 have Alzheimer's. Alzheimer's is now the 6th leading cause of death in the U.S. and there are currently no cures or methods of prevention advocated by the medical community, nor consensus on what causes this deterioration in cells and brain functioning.
But this medical community is working hard to understand exactly what is going on inside our heads. Some neuroscientists have turned to physical activity as a potential mechanism to slow damage to the brain, improve memory and executive functioning, and stimulate nerve growth. While most studies to date have been performed on mice and among older adults without Alzheimer's, some scientists are logically extending their findings to hypothesize that participating in aerobic physical activity might delay functional decline and improve brain functioning, even among people with Alzheimer's.
Now, my layperson hypothesis... This nation has seen a decline in farming, manufacturing and other physically-active occupations - slowly, since the days of the Industrial Revolution, and more rapidly over the last 30 years with the rise in information technology and the service sector of the economy. These more recent changes have also been coupled with more sedentary approaches to life and leisure - such as driving, TV-watching, internet-surfing or video game-playing - with less time for said leisure as more households rely on two or more income-earners. And more and more of the world population is living longer, which means we're using our brains longer and are reaching ages where brain-related conditions will surface among a growing proportion of the aging population (simply because the aging population continues to grow).
What if the rise in Alzheimer's and other brain functioning conditions is related to our societal shift to less physically active lives? If so, then increasing physical activity might improve brain development and prevent these debilitating and fatal conditions.
So adding a healthy dose of activity to our working (and leisure) lives might do more than strengthen our heart and prevent overweight/obesity. Why not farm (or garden) as work or leisure? Not only will it keep that brain developing, it will supply us with nutritious foods to feed brains in even earlier stages of development.