Children of older dads are more likely to be small and unhealthy, say scientists.
The controversial (to say the least) claim has been made by researchers who studied BIRDS but say their findings are also relevant to humans.
Scientists from Liverpool John Moores University found that the quality of sperm, which passes down the gene code, deteriorated as men age to produce smaller unhealthier children than those of younger males.
And they said the finding could lend weight to the idea that delaying parenthood until later life can potentially have negative consequences for the children of both older men and women.
The research focused on Houbara bustards – large birds found in Morocco, which can live for up to 23 years.
Dr Brian Peston said: "Our previous work has shown that male houbara bustards suffer declines in the number and motility of their sperm as they age, but before now it wasn't known what consequences this might have for any offspring that may be produced.
"Our key finding, was that there were also changes in the quality of offspring produced by males as they aged, in terms of their growth rate, with the highest quality chicks produced by the very youngest males.