Over-75s, on the other hand, benefited most from romps at least once a week.
Prof Shannon Shen, of Hope College, Michigan, said: "Sex is physical exercise and exercise is linked to better cognitive performance by increasing the blood flow in the brain.
"This may be especially true at older ages because of high rates of sedentary behaviour, so any circulatory benefits from having sex may be important. Although frequency of sex does decrease with age, it may be one way to combat atrophy in the brain.
"It has been shown to be beneficial for a variety of mental and physical health outcomes, including heart health, happiness and wellbeing."
Her team studied 1,683 Americans over 62 for five years.