Women approaching middle age have long been aware that the consequences
of a ticking biological clock include not only decreased fertility but also
a sharp increase in the odds of delivering a child with Down syndrome. Older
men, seemingly untouched by such biological constraints, felt free to father
children as they entered middle, and even old, age.
But now it is becoming increasingly clear that the biological clock
ticks for men as well as women, as researchers turn up evidence that as would-be
fathers get older, they have an increased chance of passing on genetic defects
to their children.
Thacker PD. Biological Clock Ticks for Men, Too. JAMA. 2004;291(14):1683-1685. doi:10.1001/jama.291.14.1683