Several months ago, my friend Anna called to complain about her boyfriend of eight months. Bombarded by media warnings about the female biological clock, he wanted to make sure that Anna was fit for childbearing before the relationship moved forward. He had taken her to a fertility clinic where a reproductive endocrinologist drew blood to check her ovarian reserve and injected radioactive iodine into her uterus to ensure that her fallopian tubes were clear.

Anna is 32. Her boyfriend is 52.

Anna's boyfriend was right to be concerned. As women increasingly pursue careers and take advantage of fertility treatments to postpone childbirth into their 30s and 40s, they do place their offspring at risk for countless disorders and diseases. This occurs, however, not because of the woman's age but because women in their 30s, like Anna, tend to couple off with older men. And when it comes to fathering healthy children, older men, it turns out, are just as much at the mercy of their biological clocks as women.

Beating the Clock Late-in-Life Fathers Larry King, 77, had his youngest at age 66. Getty Images Pablo Picasso's daughter Paloma was born when he was 67. Getty Images Rod Stewart, 66, became a father (again) in February. Getty Images

Older fathers made headlines several years ago when researchers at the Mount Sinai School of Medicine reported that a man over 40 is almost six times as likely as a man under 30 to father an autistic child. Since then, research has shown that a man's chances of fathering offspring with schizophrenia double when he hits 40 and triple at age 50. The incidence of bipolarity, epilepsy, prostate cancer and breast cancer also increases in children born to men approaching 40.

Both dwarfism and Marfan syndrome (a disorder of the connective tissue) have been linked to older fathers, and according to research published in 1996 in the journal Nature Genetics, Apert syndrome (a disorder characterized by malformations of the skull, face, hands and feet) is a mutation caused exclusively by advanced paternal age.

A 2009 study at the University of Queensland, Australia, found a correlation between advanced paternal age and poorer performance by children on intelligence tests (the children of older mothers actually performed better). And when researchers at King's College, London, bred mice from fathers of differing ages, the offspring of older fathers exhibited significant deficits in social and exploratory behavior.

Women are born with somewhere between one million and two million eggs, 75% of which are depleted by puberty. Eggs die daily—which may sound bleak—but those that endure contain an original genetic component that was created at the very start of a woman's life.

Sperm, by contrast, are more fly-by-night. After each ejaculation, a man regenerates millions of new sperm cells, and with each cellular replication, the chances rise of an error in genetic coding.

These "new" sperm might still be able to fertilize an egg, but they can contain dangerous mutations. "As men get older, maybe there is some sperm available, but a lot of that DNA may be abnormal," says Harry Fisch, author of the pioneering 2004 book "The Male Biological Clock." "After you make so many copies, the print may not be so useful."

Data are scarce on trends in paternal age, which perhaps explains why the correlation between paternal age and birth defects went undetected for so long. And, of course, "nobody likes to think that they're aging," says Dr. Fisch. "Certainly men. They were on the throne, they were the kings: 'We don't age, we stay fertile longer than women, we can have babies into our 90s....' Men live in denial."

By age 40, almost 19% of American men remain unmarried, compared to 13% of women. Whether this lag in reaching the altar is a cause or an effect of skewed dating preferences, one thing is certain: Women go for older guys. According to the online dating site OKCupid, the average 35-year-old man looks for a woman between the ages of 25 and 38, whereas a 35-year-old woman will consider dating a man between the ages of 30 and 42.

But do these women know that men have their own ticking biological clock?

Celebrities like Larry King and Rod Stewart dazzle the public with their late-in-life offspring, and it's hard not to be impressed by the occasional man in his 90s who is able to father a child. But the new research on the effects of paternal age may prompt second thoughts in 25-year-old women who would once have considered pairing off with men decades older. My friend Anna certainly had no idea that her boyfriend's age could threaten her chances of having a healthy child.

Or perhaps the new research will encourage men to take more seriously the question of when to reproduce. "We just started doing the research," says Dr. Fisch, whose goal is to get accurate information out there and to let men and women make informed choices. "The field is in its infancy. Actually, it's in utero."

—Ms. Vanderbes's most recent novel is "Strangers at the Feast," which will be published in paperback in August.