Researchers found that regular aerobic exercise boosted essential gray matter in all adults, even as young as 20 years old.
Even climbing the stairs improve the thinking skills of the students they examined, scientists said.
The positive effect of physical activity increased with age: people aged 40 seemed 10 years younger, while 60-year-olds seemed 20 years younger.
The researchers say the findings could help our understanding of non-pharmaceutical methods to balance brain aging, and could even reduce the risk of dementia.
Study author Professor Yaakov Stern, of the Taub Institute for the Research on Alzheimer's Disease and the Aging Brain at Columbia University in New York, said: 'As people age, there can be a decline in thinking skills.
'However our study shows getting regular exercise may help slow or even prevent such decline.
'We found all participants who exercised not only showed improvements in executive function but also increased the thickness in an area of the outer layer of their brain.'
The study published in Neurology found exercise specifically improved neurons in areas that control executive function.
These relate to a person's ability to regulate their own behavior, pay attention, organize and achieve goals.
Dr Stern and colleagues followed 132 people aged 20 to 67 who were randomly assigned to six months of either aerobic exercise or stretching and toning four times a week.
Those in the former group chose from activities including walking on a treadmill, cycling on a stationary bike or using an elliptical machine that simulates stair climbing.
The researchers found these were twice as effective as stretching and toning. Aerobic exercisers improved their overall scores on executive function tests by 0.50 points.
This was a statistically significant difference from those who did stretching and toning - who progressed by 0.25 points.
At 40, the improvement was 0.228 standard deviation units higher in those who exercised compared to those who did stretching and toning. At 60 it was 0.596 more.
Dr Stern said: 'Since a difference of 0.5 standard deviations is equivalent to 20 years of age-related difference in performance on these tests, the people who exercised were testing as if they were about 10 years younger at age 40 and about 20 years younger at age 60.'
He added: 'Since thinking skills at the start of the study were poorer for participants who were older, our findings suggest that aerobic exercise is more likely to improve age-related declines in thinking skills rather than improve performance in those without a decline.'
None of the participants smoked or had dementia. They also did not exercise at the start of the study and had below average fitness levels.
The aerobic and stretching groups were equally balanced for age, sex, education as well as memory and thinking skills at the outset.
They worked out at a fitness center and checked in weekly with coaches monitoring their progress. They all wore heart rate monitors as well.
Thinking and memory skills were evaluated at the start as well as at three months and at the end of the six-month period.
Participants in the exercise group ramped up their activity during the first month. During the remainder of the study they trained at 75 percent of their maximum heart rate.
People in the stretching and toning group did exercises to promote flexibility and core strength.
Researchers measured participants' aerobic capacity using a cycling machine called an ergometer that estimates exercise intensity. Participants also had MRI brain scans at the start and end.
This identified an increase in the thickness of the outer layer of the brain in the left frontal area in all the aerobics group - suggesting it contributes to brain fitness at all ages.
'Our research confirms that exercise can be beneficial to adults of any age,' said Dr Stern.
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