Children with older fathers have a significantly increased risk of having autism, a study has concluded.
Having an older father has been linked to other conditions
The UK and US researchers examined data on 132,271 children and said those born to men over 40 were six times more at risk than those born to men under 30.
They said the study in Archives of General Psychiatry was further proof men also had "biological clocks".
One UK expert said the study could be important in understanding the genetic mechanisms underlying autism.
Autism and related conditions, known as autism spectrum disorders, have become increasingly common, affecting 50 in every 10,000 children as compared with five in 10,000 two decades ago.
Increased awareness and changes in the way the disorders are diagnosed are thought to play a major role in the increase, but the researchers say it may also be linked to other changing factors.
Older parental age has previously been linked to abnormalities in the brain development of children.
The researchers, from Mount Sinai School of Medicine, New York, and the Institute of Psychiatry at King's College London, looked at data on 132,271 children born in Israel during the 1980s.
All men, and three-quarters of women born in these years were assessed by the draft board at age 17, during which time any disorders were recorded.
The board also took information on the ages of their father and mother, and took into account factors such as year of birth and socioeconomic status.
Among those whose fathers were between 15 and 29 when they were born, the rate of autism was six in every 10,000, rising to nine in every 10,000 when fathers were aged 30 to 39 (1.6 times higher).
In the group whose fathers were aged 40 to 49, the rate rose to 32 in 10,000 (5.75 times higher).
The rate appeared to be even higher when fathers were aged over 50, but the researchers said the sample size was very small.
The mother's age did not appear to influence the chances a child would have autism.
The researchers suggest there may be a genetic fault which is more common with age.
This might be spontaneous mutations in sperm-producing cells or alterations in genetic "imprinting," which affects gene expression.
The team, led by Dr Avi Reichenberg from the IoP, said: "It is important to keep in mind that age at paternity is influenced by the socio-cultural environment and varies across societies and over time.
"In a given population, a change in the socio-cultural environment could produce a change in paternal age at birth.
"In theory, it could thereby lead to a change in the incidence of genetic causes of autism."
He added: "Although further work is necessary to confirm this interpretation, we believe that our study provides the first convincing evidence that advanced paternal age is a risk factor for autism spectrum disorder."
Professor Simon Baron Cohen, of the Autism Research Centre in Cambridge, said: "The finding of a significant association with advancing paternal age is one that should be straightforward to test in other samples, to see if this result from a purely Israeli sample generalises to other populations.
"If confirmed, it could have important implications for the genetic mechanisms underlying autism."