MONDAY, March 9 (HealthDay News) -- Men who put off becoming dads till later in life may pay a price: slightly lowered intelligence in their offspring.

That's the conclusion of an Australian study that found that kids born to older men underperformed on intelligence and cognitive tests from infancy to 7 years of age, compared with children of younger fathers.

But on the other hand, children born to older mothers scored higher on the same tests, the team said.

"The biological clock ticks for men, too," concluded Dr. Mary Cannon, an associate professor of psychiatry at the Royal College of Surgeons in Dublin, Ireland, and the author of an accompanying editorial in the March issue of the online journal PLoS Medicine.

"There are risks associated with delaying fatherhood," she said. "These risks may be subtle, such as a decrement of three to six points on childhood IQ tests, but can also be significant, as in the increased risks of serious mental illnesses like schizophrenia and autism."

One reason may be that men's sperm change as they age, the Australian researchers suggested.

"We suspect that more mutations accumulate in sperm as the dads age," said Dr. John McGrath, from the Queensland Brain Institute at the University of Queensland in Brisbane, Australia, and the study's lead researcher. "These mutations may cause subtle changes in the way the brain develops. But other social factors are involved also."

For the study, McGrath's team collected data on more than 33,000 American children born between 1959 and 1965. The data, which came from the U.S. Collaborative Perinatal Project, included the children's cognitive test results at the ages of 8 months, 4 years and 7 years. The tests included assessments of sensory discrimination and hand-eye coordination, conceptual and physical coordination and, at age 7, reading, spelling and arithmetic skills.

In addition, the researchers took into account socioeconomic factors, including family income.