"But beyond that age and into adulthood there is a difference of five points, which is small but it can have important implications," he said.
"This is against a background of women dramatically overtaking men in educational attainment and making very rapid advances in terms of occupational achievement."
The academics used a test which is said to measure "general cognitive ability" - spatial and verbal ability.
As intelligence scores among the study group rose, the academics say they found a widening gap between the sexes.
There were twice as many men with IQ scores of 125, for example, a level said to correspond with people getting first-class degrees.
At scores of 155, associated with genius, there were 5.5 men for every woman.