While older women are said to have a higher risk of having babies with birth defects, it has long been presumed that men can have children at any age. But a new study has shown that it's the other way round.

Researchers in Japan have found that older fathers are twice as likely to have autistic children as younger men while the mother's age has no influence as such on the likelihood of her offspring developing the disorder, the 'British Journal of Psychiatry' reported.

They have based their findings on an analysis of 84 kids with high-functioning autism spectrum disorders, meaning they had social impairments of the condition -- but had normal intelligence, and 208 children without the disorder.

Children whose fathers were over 33 years were 1.8 times more likely to have autism than those fathers were under 29. And men who fathered children between the age of 29 and 32 years were 30 per cent more likely to have an autistic child, the study found.

But experts are not fully convinced.

"The causes of autism are still being investigated. Many experts believe the pattern of behaviour from which autism is diagnosed may not result from a single cause. Autism affects around one in 100 people in the UK and does not solely affect children of older parents.

"Members of NAS are made up of parents of children from a variety of ages and backgrounds; in addition there is evidence to suggest complex genetic factors were responsible for some autism forms," 'The Daily Telegraph' quoted Benet Middleton of National Autistic Society as saying.