A study of Danish families has concluded that children born to a parent over 35 have an increased risk of developing an autism spectrum disorder - regardless of whether one or both the parents are older.
Erik Thorlund Parner from the University of Aarhus School of Public Health in Denmark led the study and claimed that if genetic problems arising from older sperm or eggs explained the findings, then having both an older sperm and an older egg together should mean an even higher risk of autism for the child.
Despite this, Parner and his research team did not find a higher risk of autism among youngsters with two older parents compared with just one.
The study looked at data collected from over 9,500 children in Denmark who had already been diagnosed with an autism spectrum disorder, ranging from mild Asperger's through to severe social disability.
Speaking to Reuters Health, Mr Parner said the result was 'surprising':
"The result was surprising in that there was no additive effect of maternal and paternal age," he said, adding "We do not really have a qualified guess on the explanation to the non-additive effect. I don't believe that one can yet say anything conclusive about whether (having an) older parent is biologically related to autism."
Previous studies have also concluded that older mums and dads are more likely to have a child with autism, but the findings have never been consistent, with results ranging from an older mother increasing the risk, through to an older father being the cause if the mum is under 30.
Mr Parner said the study will lead to a larger analysis of autism taking in dating from around the world.