This is also a time when the central nervous system is still developing, he noted.
The study was an extremely large one, involving 1.2 million Swedish men born between 1950 and 1976.
More than 250,000 of the men were sibling pairs and more than 3,000 were twins, of which 1,432 were identical twins.
The researchers took information from the time the men were conscripted into the military (age 18), which is compulsory in Sweden. This information was then correlated with information on the men's prior academic performance, how many siblings they had and what socioeconomic class they came from.
Better cardiovascular fitness was associated with higher intelligence, although muscle strength was not, the researchers found.
"The emphasis at gyms is for strength over aerobic capacity but aerobic fitness is where we need to pay our focus," said Dr. Jonathan H. Whiteson, co-director of the Joan and Joel Smilow Cardiopulmonary Rehabilitation and Prevention Center at NYU Langone Medical Center in New York City.
The twins portion of the analysis showed that environmental factors accounted for more than 80 percent of the equation, and genetics for less than 15 percent.
Any number of factors could explain why better cardiovascular fitness through exercising impacts brain function, including improved blood flow to the brain, diminished anxiety, enhanced mood and less fatigue, Whiteson said.
"We've known that aerobic exercise has been associated with improved cognitive performance. We've known that from studies dating back from the '70s," he said. "They picked the area of young men which may not have had a lot of research. This confirms what we've already known about younger and older individuals."
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