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A wide range of disorders and problems in school-age children have been linked to delayed fatherhood in a major study involving millions of people.

Increased rates of autism, attention deficit hyperactivity disorder (ADHD), bipolar disorder, schizophrenia, suicide attempts and substance abuse problems were all reported.

The study, in JAMA Psychiatry, suggests mutated sperm were to blame.

However, experts say the benefits older dads bring may outweigh any problems.

The investigation by Indiana University, in the US, and Sweden's Karolinska Institute has been described as the largest and one of the best designed studies on the issue.

The researchers looked at 2.6 million people and at the difference between siblings born to the same father as it accounts for differences in upbringing between families.

Comparing children of a 45-year-old dad to those of a 24-year-old father it indicated:

There was no starting point after which the risk started to increase, rather any increase in age had an associated increase in risk.

However, the overall risk is likely to remain low. Even if a the risk of a rare disorder is doubled, it is still very unlikely to affect a child.

The problem is small increases in risk scaled up over millions of people having children later can lead to an increased incidence of such disorders.

One of the researchers, Dr Brian D'Onofrio, said he was shocked by the findings, which suggested a higher risk than previously estimated.

He told the ³ÉÈË¿ìÊÖ: "The implications of the study is that delaying childbearing is also associated with increased risk for psychiatric and academic problems in the offspring.

"The study adds to a growing body of research, that suggests families, doctors, and society as a whole must consider both the pros and cons of delaying childbearing."

Sperm are produced constantly throughout a man's lifetime. As the sperm-making mechanism ages, so too do the number of errors - older sperm have more mutations which may be damaging.

Dr James MacCabe, senior lecturer in psychosis at the Institute of Psychiatry: "I would make the point very strongly that men should not decide on whether or when to have children on a single study, or cumulative studies."

He said the risks were low and that even a doubling or trebling of risk would still affect a small proportion of people.

"Having said that, with the demographic change we have seen in the last decade, on a population level this is a concern and we might expect higher rates of psychoses now and in the future."

He added that older dads bring many advantages such as more stable relationships and higher income, which "probably outweigh" any risks.