More men are becoming fathers later in life than ever before. In the U.S., the average age for first-time fathers has climbed to about 31,1 and men aged 50 and older now make up 1.3% of all fathers — a small but clear increase from 1.1% just a decade ago.2 Careers, financial stability, evolving relationships, and personal readiness all play a part in why many people are choosing to delay parenthood.
For years, the “biological clock” has been framed almost exclusively as a woman’s issue, compromising egg quality, raising chromosomal risks, and narrowing the window for conception. Men, on the other hand, were viewed as fertile for life simply because sperm production never stops. Yet research increasingly shows that a man’s reproductive system changes with age as well, in ways that science is only beginning to understand better.
New Study Maps the Genetic Toll of Aging Sperm
In October 2025, researchers from the Wellcome Sanger Institute and King’s College London published a landmark study in Nature that traced how harmful genetic mutations accumulate in sperm as men age. Conducted in collaboration with the TwinsUK study — the largest adult twin registry in the U.K. — the research aimed to determine how a man’s age shapes the genetic material he passes on to his children.3
• Sequencing revealed how mutations build and spread with age — The team analyzed sperm samples from 81 healthy men aged 24 to 75, using high-precision DNA sequencing to track how tiny genetic changes accumulate over time. The results showed that sperm mutations do not simply result from age-related wear and tear. Instead, certain mutations are favored during sperm production, giving some sperm-producing cells a biological edge over others.
• “Selfish” sperm cells dominate over time — These advantaged cells divide faster and outcompete neighboring ones, passing their mutations to a larger share of sperm. Over decades, this creates a quiet evolutionary process that reshapes the genetic landscape of the testes, favoring mutations that replicate efficiently, even when they increase disease risk for the next generation.
• Researchers identified 40 genes where these mutations tend to occur — Many of these genes are tied to neurodevelopmental and childhood disorders such as autism, epilepsy, and congenital heart disease, as well as certain cancers. While 13 of these genes were previously known, the study showed that the phenomenon is far more widespread, affecting genes vital for cell growth and development.
• Harmful DNA changes not only persist but also multiply with age — Sperm from men in their early 30s carried disease-causing mutations in about 2% of cases, while men aged 43 to 74 showed rates of 3% to 5%. By age 70, roughly 4.5% of sperm contained damaging mutations. Not every mutation leads to conception; some prevent fertilization, hinder embryo development, or cause miscarriage. Still, the trend reveals a measurable increase in genetic risk as paternal age rises.
• These findings redefine how we view the male germline — The germline is the lineage of cells that produces sperm and carries genetic information to future generations. The study demonstrated that natural selection acts at the cellular level within the testes, an organ once considered genetically stable. According to Dr. Raheleh Rahbari, senior author and group leader at the Wellcome Sanger Institute:
“There’s a common assumption that because the germline has a low mutation rate, it is well protected. But in reality, the male germline is a dynamic environment where natural selection can favor harmful mutations, sometimes with consequences for the next generation.”4
• Findings confirmed across large-scale genetic data — In a complementary Nature study,5 scientists from Harvard Medical School and the Sanger Institute analyzed over 54,000 parent-child trios and 800,000 healthy individuals.
They found similar signs of positive selection in more than 30 genes, confirming that the same mutations expanding within the testes are being transmitted to offspring. This cross-validation between sperm- and family-based data represents a major step forward in understanding how paternal age shapes genetic inheritance.
Taken together, these studies show that while men remain capable of fatherhood later in life, age brings measurable biological changes that affect both fertility and the health of future children. Recognizing how these changes occur gives you the awareness to plan ahead and make choices that support long-term reproductive health.
Earlier Evidence on Age-Related Mutations of the Male Germline
Before the 2025 Nature report, scientists had already begun mapping the genetic fingerprints of aging in the male reproductive system. For instance, in 2022, a comprehensive review published in Fertility and Sterility compiled decades of research showing how sperm changes with advancing age, both in its physical characteristics and its genetic integrity.6
• Age alters sperm structure and function — The review summarized consistent declines in sperm count, motility, and normal shape (morphology) with age. Beyond these visible changes, the most significant findings came from the genetic level, where age-related mutations in sperm DNA steadily increase over time.
• De novo mutations rise with paternal age — Data across multiple studies showed that genetic changes appearing for the first time in offspring (known as de novo mutations) become more frequent as men get older. These mutations have been linked to a higher risk of neurodevelopmental conditions such as autism and schizophrenia, and certain congenital anomalies.
• Stem cell aging explains mutation buildup — Earlier work on spermatogonial stem cells, the source of all sperm, revealed that each cell division introduces small DNA copying errors. Over years of continuous division, these errors accumulate, raising the mutational load within the germline. Some mutations even grant growth advantages to particular stem cell lines — supporting the “selfish” selection process that the 2025 Nature study later confirmed in detail.
• Clinical outcomes mirror molecular findings — The review connected these molecular changes to clinical observations. Men of advanced paternal age were shown to have a higher likelihood of fathering children affected by autism spectrum disorder, achondroplasia, schizophrenia, or certain childhood cancers. The underlying mechanisms included both increased mutation rates and alterations in how sperm DNA is packaged, which influenced the information delivered to the embryo.
• Epigenetic drift affects gene regulation in offspring — The review also pointed to shifts in DNA methylation and other epigenetic markers that control how genes are switched on or off. These modifications were found to influence early embryonic development.
These results support the findings that male fertility is not fixed, and the biological clock, once considered a woman’s concern, quietly ticks for men as well.
Beyond Age — Other Threats to Male Fertility
While age plays a key role in sperm quality, modern environmental and lifestyle pressures are eroding male fertility even in younger men. Analyses spanning five decades reveal that average sperm counts worldwide have dropped by more than half since the 1970s. Researchers point to a convergence of chemical, physical, and behavioral stressors that collectively undermine sperm health.7
• Endocrine-disrupting chemicals (EDCs) — These synthetic compounds mimic or block natural hormones, disrupting signaling in the testes and skewing the balance of testosterone and estrogen. The result is lower sperm count, poorer motility, and more abnormal sperm.
EDCs, found in plastics, personal-care products, food packaging, and even household dust, include bisphenol A (BPA), phthalates, and per- and polyfluoroalkyl substances (PFAS), making daily exposure nearly unavoidable.8
• Microplastics — Tiny plastic fragments less than 5 millimeters across have been detected in human and animal testes. In one 2024 study of 47 dogs and 23 men, researchers identified 12 types of microplastics in all samples, with polyethylene the most common.9
Human testes contained an average of 328 micrograms per gram, which is nearly three times that of dogs. These particles trigger inflammation and oxidative stress, and impair testosterone levels, exacerbating the hormonal disruption already caused by embedded EDCs.
• Obesity and poor nutrition — Excess body fat and processed-food diets amplify oxidative stress and reduce sperm quality. Overweight men are about three times more likely to have sperm counts below 20 million per milliliter compared to men with a healthy weight.10
Obesity suppresses free testosterone, raises scrotal temperature, and promotes systemic inflammation, while high intakes of refined sugars and vegetable oils further damage sperm DNA and reduce motility.11,12
• Stress — A systematic review published in Reproductive Biology and Endocrinology found that psychological and occupational stress consistently lowered sperm concentration, motility, and morphology, and increased DNA fragmentation. Researchers concluded that sustained stress disrupts the body’s hormonal balance and testicular function, even in otherwise healthy men.13
• Smoking and alcohol — Cigarette smoke exposes sperm to more than 7,000 toxins, many of which directly damage sperm membranes and DNA. A 2023 review in Frontiers in Endocrinology found that heavy smokers had significantly lower semen volume, sperm concentration, and motility, as well as a higher proportion of morphologically abnormal sperm compared to nonsmokers.14
Similarly, alcohol consumption has been strongly linked to reduced semen quality and impaired hormonal regulation. According to a study published in the International Journal of Environmental Research and Public Health, chronic drinking induces oxidative stress, disrupts testosterone regulation, and impairs the cells responsible for sperm production. Even moderate, long-term intake has been linked to reduced sperm count and motility.15
• Electromagnetic fields (EMFs) — Continuous exposure to mobile phones and Wi-Fi radiation has been linked to increased DNA fragmentation and lower sperm motility and viability. Carrying a phone in a pants pocket heightens oxidative stress in the testes and damages Leydig cells, which produce testosterone.16
• Air pollution — Airborne pollutants, including heavy metals, particulate matter, and polycyclic aromatic hydrocarbons, disrupt hormone regulation and are associated with reduced sperm counts and altered sperm morphology. While exposure varies geographically, air quality is now recognized as a measurable determinant of male reproductive health.17
Understanding how these pressures accumulate gives you a clearer view of the modern fertility challenge and where protection needs to begin. Learn more about the factors influencing your reproductive health in “Lifestyle Choices Are Sabotaging Male Fertility.”
Strategies to Protect Your Fertility Before It’s Too Late
While age can’t be changed, your daily choices still matter. Taking care of your body during your prime years helps preserve fertility and maintain healthier sperm for longer. Below are key actions to help you protect your fertility:
1. Maintain a healthy weight — Exercise on a regular basis and build your meals around whole foods. Healthy carbohydrates should make up the bulk of your meals, ideally from whole carb sources like ripe fruits and well-cooked vegetables. Most adults need approximately 200 to 250 grams of carbs per day.
Moreover, you should consume about 0.8 grams of protein per pound of your ideal body weight, with one-third coming from collagen sources like bone broth or grass fed ground beef. Replace vegetable oils rich in linoleic acid (LA) with healthy fats such as butter, ghee, and coconut oil. I also recommend using tools like Food Buddy, which is a key feature of my upcoming Health Coach App, to balance your macronutrient intake.
2. Minimize exposure to EDCs and other environmental toxins — To reduce your exposure to EDCs, start by replacing plastic containers with glass, ceramic, or stainless steel, and stop heating food in plastic altogether.
When buying fruits and vegetables, choose organic varieties, ideally from your local farmers market, and wash them thoroughly. You should also look for unscented household and personal care products, and filter your tap water. For more tips, check out this article.
3. Avoid EMF exposure — Be mindful of where and how you use electronic devices. Keep your phone and tablet off your body, use wired headphones or speaker mode for calls, and avoid storing devices in your pockets. Place laptops on desks instead of your lap, and turn off Wi-Fi routers overnight. At home, keep devices out of your bedroom to give your body a nightly break from constant exposure.
4. Manage stress and support restorative sleep — Incorporate stress-reduction techniques into your daily routine, such as meditation, yoga, deep breathing exercises, or engage in activities you enjoy. You can also simply step outside for fresh air, stretch every few hours, or take short breaks between tasks.
In the evening, give your body clear signals that it’s time to rest — dim the lights an hour before bed, keep your room cool and dark, and go to sleep at the same time each night. Get more tips in “Top 33 Tips to Optimize Your Sleep Routine.”
5. Consider supplementing for sperm health — Several nutrients have been shown to enhance sperm quality, particularly CoQ10, L-carnitine, zinc, selenium, vitamin E, and omega-3 fats.18 These compounds support mitochondrial function, improve sperm motility, and protect DNA from oxidative damage. While a healthy, diverse diet should provide most of your nutrient needs, targeted supplementation may offer additional support.
6. Consider medical screening and sperm preservation early — If you plan to delay fatherhood, a semen analysis provides valuable insight into sperm count, shape, and motility. Repeat testing over time helps you monitor changes.
If you expect to delay fatherhood, talk to a fertility specialist about sperm banking. Collecting and storing samples while your fertility is optimal ensures that genetic material is preserved even if sperm quality decreases later in life.
Protecting your fertility is a long-term investment in your well-being. The effort you put into today shapes not only your future family but the quality of your health for years to come.
Frequently Asked Questions (FAQs) About Male Fertility and Aging
Q: At what age does a man’s fertility really start to decline?
A: Research shows that sperm quality begins to change when a man reaches his 30s. By the early 40s, sperm count, motility, and DNA integrity show measurable decline. These changes don’t make fatherhood impossible, but they do increase the time it may take to conceive and slightly raise risks of genetic abnormalities in offspring.
Q: What actually happens to sperm as men age?
A: Each time sperm-producing cells divide, small DNA copying errors can occur. Over decades, these mutations accumulate and can be passed to the next generation. Researchers found that certain “selfish” mutations even give sperm-producing cells a growth advantage, allowing them to multiply faster and spread harmful genetic variants over time.
Q: Is there an age when men need to seriously think about preserving sperm?
A: If you plan to delay fatherhood beyond your late 30s, it’s worth discussing sperm preservation with a fertility specialist. Banking sperm while you’re younger allows you to secure genetic material that’s less affected by age-related mutations. It’s a proactive step, especially for men exposed to environmental toxins, medical treatments, or high stress.
Q: What are the other factors that might affect male fertility?
A: Beyond age, sperm quality is shaped by lifestyle and environmental exposures. Endocrine disruptors from plastics, pesticides, and household products can alter hormone balance. Microplastics found in human testes may trigger inflammation and oxidative stress. Obesity, poor diet, smoking, alcohol, stress, air pollution, and constant EMF exposure also contribute to lower sperm count and quality.
Q: What can I do to protect my reproductive health?
A: Stay active and maintain a healthy weight through consistent movement and nutrient-rich meals. Cut out sources of linoleic acid and avoid ultraprocessed foods to support hormonal and metabolic health. Reduce daily toxin exposure by storing food in glass or stainless steel instead of plastic, choosing organic produce when possible, filtering your tap water, and keeping phones and other electronic devices away from your body.
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