Researchers in the U.S. say that even though men are able to father children well into old age, the quality of the sperm declines as men age.
It seems not only does the sperm lose their ability to swim in a straight line they also become genetically defective.
These latest results support other recent studies which have found that men become less fertile and also tend to have more children with birth defects as they age.
The study was led by scientists at Lawrence Livermore National Laboratory (LLNL) and the University of California, Berkeley.
Andrew Wyrobek co- lead author says it shows that men who wait until they're older to have children are not only risking difficulties conceiving, they could also be increasing the risk of passing along dwarfism and other genetic diseases to their children.
The apparent increase in birth defects with aging has traditionally been attributed to women, whose fertility drops considerably with age and disappears totally at menopause.
Women are born with all their eggs and as they age the egg cells mature and ripen and older eggs are often defective.
This affects female fertility and creates the likelihood of genetic defects such as Down syndrome.
Older women are known to have an increased risk for infertility, spontaneous abortion, and genetic and chromosomal defects in pregnancy but the affects of male aging is less well researched say the researchers.
The research suggests that men also have a biological time clock.
For the study, the researchers analyzed DNA damage, chromosomal abnormalities and gene mutations in semen samples from 97 healthy, non-smoking LLNL employees and retirees between 22 and 80 years old by using a variety of state-of-the art methods for detecting defects in human sperm.
The researchers gathered extensive medical, lifestyle and occupational exposure history from the men and disqualified from the study those who were cigarette smokers and men with current fertility or reproductive problems or who had undergone chemotherapy or radiation treatment for cancer.
They then examined the sperm for features such as the ability to swim quickly, and also for DNA fragmentation, which is a measure of damage linked with male fertility, successful conception, and sustained pregnancy.
The researchers found that men usually started to have an abnormal DNA fragmentation index at the age of 56 and they say that their findings provide further evidence that men choosing to delay fatherhood may have a lower likelihood of a successful pregnancy free of early loss and gene defects.
But they also found that there did not appear to be a reliable test for healthy sperm and men with apparently good semen could still be at risk of fathering a child with a genomic defect.
The study also identified a small number of men who may be at increased risk for transmitting multiple genetic and chromosomal defects.
They say it is important to understand the effects of men having children at ever-older ages as increasing numbers of older men are having children.
Since 1980 there has apparently been a 40 percent increase in 35- to 49-year-old men fathering children, and a 20 percent decrease in fathers under 30.
Earlier studies have also shown that it takes longer for older men to conceive, even when the age of the mother is considered.
The report is published in the current issue of the Proceedings of the National Academy of Sciences.