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The concept of the female biological clock is no secret. Seemingly everyone knows—and likes to remind women of a certain age (trust me) that the bell starts to toll towards the mid-thirties, gets louder and more urgent in the late thirties and early forties, and falls completely silent at menopause, around age 50. The male biological clock, on the other hand, is rarely mentioned, and why should it be? After all, Al Pacino had a baby at 83, Robert De Niro at 79, Mick Jagger at 73. It’s easy to wonder: is there even such a thing as the male biological clock?

The short answer is yes, but it doesn’t look anything like the fairer sex's. “The big difference is that on the female side, a woman is born with all the eggs she'll ever have throughout her life. And on the male side, there is this constant turnover production of sperm, because the testicular tissue is making sperm each and every day,” says , a fertility doctor at CCRM Fertility. While women eventually reach the end of their finite egg supply, men continue this daily sperm production throughout life. “You can see a 90-year-old guy in a wheelchair rolling down the hall, and he's making fresh new sperm every day,” says , incoming Assistant Urology Professor at the USC Keck School of Medicine.

This doesn’t mean, however, that the sperm of a 90-year-old man is just as healthy as the sperm of a 25-year-old man, or that older men won’t have more trouble achieving a healthy pregnancy or producing healthy offspring than younger men. “Aging still has significant implications in terms of male reproductive and sexual health,” says Dr. Asanad.

The Effects of Aging on Male Fertility

To begin with, the sperm of an older man does not look the same under a microscope as that of a younger man. “Semen analyses show significant changes over time as men age,” says Dr. Hanson. According to  referenced by Dr. Asanad, sperm volume decreases by 3-22%, sperm motility (the ability of the sperm to move) worsens by 3-37%, and rates of abnormal morphology (the size and shape of the sperm) increase by 4-18% as men age. These declines are mirrored by changes “on the molecular, genetic level—we can see higher rates of damaged DNA inside individual sperm cells as men get older,” says Dr. Hanson.

These changes impact fertility in a number of ways. A decrease in semen volume can lead to a decrease in sperm count, resulting in lowered odds of conception. Worsened sperm motility likewise reduces the odds of conception, as it means sperm are swimming less efficiently through the female reproductive tract—they may be swimming more slowly, in circles, or not moving forward at all. And increased rates of abnormal morphology mean there will be more abnormally shaped sperm approaching the egg, and those sperm tend to have difficulty penetrating the egg in order to fertilize it. Sperm DNA damage above a certain threshold, meanwhile, is associated with recurrent miscarriage, recurrent IVF failure, and poor fertilization of eggs.

Beyond these changes in sperm quality, the other relevant shift men experience as they age is in testosterone levels, which are a critical component of fertility. “Endogenous testosterone production, meaning from within a man's own body from within their testicles, plays a huge role in sperm production,” says Dr. Asanad, who adds that it can additionally impact sperm quality and function beyond sperm count. “Men with low testosterone also have decreased libido, decreased frequency of intercourse, and they may have erectile dysfunction—all of which play a role in fertility,” says Dr. Asanad.

With that said, just how much testosterone decreases as men age is up for debate. Dr. Asanad points out that early  showed men over the age of 60 had 20% lower testosterone levels, men over the age of 70 had 33% lower testosterone levels, and men over the age of 80 had 50% lower testosterone levels. “Over the years, these findings have become controversial,” he says. “There have been a few studies showing that healthy aging men in their 50s or 60s are actually able to maintain normal testosterone levels.” This suggests that declining testosterone levels have more to do with developing comorbidities, such as high blood pressure and diabetes, than with age itself. Regardless of the direct cause, however—whether it’s aging or disease—Dr. Asanad says that older men generally do tend to have lower testosterone levels than younger men.

The Timeline for Fertility Decline

Another major difference between male fertility and female fertility is that it’s much more difficult to assign an age at which these impacts occur in men. Because women have a finite number of eggs, and it’s well established that egg quality decreases in women over the age of 35, says Dr. Asanad, it’s much easier to identify female fertility windows and to assign them a ‘biological clock.’ With male fertility, this is harder to do, and not only because changes happen more gradually. “When you look at studies evaluating [fertility metrics in] men of older ages, some of those studies use men aged 35 or above, and some use 40, 45, 50, 55, even up to 60 as thresholds for what is considered advanced paternal age,” Dr. Hanson says. “That creates a muddied picture when we try to standardize findings, because there's no set definition [of advanced paternal age].” If he had to give that threshold a number, however, Dr. Hanson says the decline begins to accelerate after the age of 40.

While many men are able to conceive despite declining fertility, age-related changes do result in infertility in some men. “We define infertility as not being able to achieve a pregnancy within 12 months, and research shows that guys who are slightly older have a decreased chance of achieving a pregnancy within that year,” says Dr. Asanad. For example, “there was  that compared guys who are over the age of 35 to guys who were under the age of 25, and it found that guys who were over 35 had a 50% lower chance of achieving a pregnancy within 12 months.

Given the above data, it’s no surprise that male infertility is more common than you might think. According to , Men’s Sexual Health & Reproductive Urologist, infertility occurs in around 10% of reproductive-age couples in the US, and Dr. Hanson says that male infertility is the sole cause of a couple’s inability to conceive around 30% of the time, female infertility is the sole cause of a couple’s inability to conceive around 30% of the time, and the rest of the time it’s due to a combination of factors from both sides. In other words, “you’re equally likely to encounter male problems as you are to encounter female problems,” he says.

Considerations Beyond the Ability to Conceive

The ability or inability to conceive isn’t the full picture when it comes to male fertility. Even when men can conceive into advanced paternal age, certain risks to offspring increase. “Conditions that become more likely in children born to fathers who are advanced in age include ,��,��, and other types of neuropsychiatric disorders where mental functioning perhaps isn't ideal [e.g. ���ǰ���],” says Dr. Hanson, going on to point out that certain pediatric leukemias and other pediatric cancers have been linked to advanced paternal age.

The stats here can be alarming. For example,��, which looked at over 300,000 men, found that men over 40 were 5.75 times more likely to have offspring with autism spectrum disorder than younger men, even when controlling for other factors, such as the age of the female partner and socioeconomic status. Other  showed that men between the ages of 45 and 49 are twice as likely than those under age 25 to have children with schizophrenia, and that for men over the age of 50, that risk increases threefold.

The age of the male partner can also impact pregnancy and birth outcomes. One  out of Columbia University showed that women, no matter how young and healthy, were more likely to miscarry the older their male partners were.  showed that men over 45, independent of their partner’s age, had 14% higher odds of premature birth than younger men. Additional  from 2005 concluded that older fathers played a larger role in IVF pregnancy failure than had been previously understood.

Paternal age may also increase the risk of conditions such as �����Ի���gestational diabetes in female partners; however, Dr. Hanson caveats that research around the impact of paternal age on these specific conditions—unlike the research done on the impacts on offspring—is sparse, and hasn’t as clearly teased out whether increased prevalence is due to maternal or paternal age.

In terms of the age at which men should start to worry about these increased risks, the problem of non-standardized definitions of advanced paternal age once again factors in, making it difficult to pinpoint exact tipping points. Dr. Asanad says that population studies suggest that risk increases after the age of 45, and sometimes even after the age of 40; however, it can be difficult to tease out whether or not that means a significant increase in risk happens at 45, 55, or beyond. “In my opinion, the inflection point, where that risk goes from a slow linear increase to a more logarithmic rapid rise, is about age 65 in males,” says Dr. Turek. In other words, risk rises slowly over time, and then more dramatically at around age 65.

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