New York:
People who marry a partner 10 or more years younger are at higher risk of having autistic children, according to the largest-ever multinational study of parental age and autism risk.
The study which included 5.7 million children in five countries also found a higher risk of autism among children of older parents and those born to teen parents.
"Although parental age is a risk factor for autism it is important to remember that, overall, the majority of children born to older or younger parents will develop normally," said co-author Sven Sandin, a medical epidemiologist affiliated with the Icahn School of Medicine at Mount Sinai, in New York, and Sweden's Karolinska Institutet.
The study looked at autism rates among 5,766,794 children - including more than 30,000 with autism - in Denmark, Israel, Norway, Sweden and Western Australia.
The children were born between 1985 and 2004, and the researchers followed up on their development until 2009, checking national health records for autism diagnoses.
According to the researchers, autism rates were 66 per cent higher among children born to fathers over 50 years of age and 28 per cent higher when fathers were in their 40s, than among those born to fathers in their 20s.
They also found that autism rates were 18 per cent higher among children born to teenage mothers than among those born to mothers in their 20s.
Autism rates were 15 per cent higher in children born to mothers in their 40s, compared to those born to moms in their 20s.
Autism rates rose still higher when both parents were older, in line with what one would expect if each parent's age contributed to risk.
The study also found that autism rates increased with widening gaps between two parents' ages.
These rates were highest when fathers were between 35 and 44 and their partners were 10 or more years younger. The rates were also high when mothers were in their 30s and their partners were 10 or more years younger.
The higher risk associated with fathers over 50 is consistent with the idea that genetic mutations in sperm increase with a man's age and that these mutations can contribute to the development of autism spectrum disorder (ASD).
By contrast, the risk factors associated with a mother's age remain unexplained, as do those associated with a wide gap between a mother and father's age, researchers said.
The study was published in the journal Molecular Psychiatry.