Old women can't have kids. Older mothers risk more birth defects in kids. These facts are well known. Now research is accumulating that older men are less likely to have kids at all, and older fathers are more likely to have kids with birth defects. The numbers are not as dramatic as for women, and the risks are not as great. Eggs, after all, are not made new, but keep aging from the mother's birth. New sperm are created all the time. But the organs for making sperm keep aging, and they wear down and make mistakes more often in later years.
To put some numbers to these risks, the children of younger fathers run a 3% risk of birth defects. With fathers over 45, the risk rises to about 4%. This is still not a large risk, but a huge percentage increase. Likewise, autism is rare, but there seems to be more of it than there used to be. One reason might be that older fathers are more than 5 times as likely to produce autistic children compared to younger (under 30) fathers. One of the most specific results reported comes from a large-scale Israeli study of schizophrenia, which found a risk of 1 in 141 in children of fathers under 25, 1 in 99 for fathers 30 to 35, and 1 in 47 for fathers 50 and older.
The research on the fertility of older fathers is just beginning, compared to studies of older mothers. There is not even the same level of speculative theory about why male fertility declines in number and quality. Still, I think it is already clear that both men and women – and their children – are better off if a couple can have their kids in their mid-twenties to mid-thirties.