Yes, more gradually than for women, and it starts to matter from the mid-30s. A 2015 systematic review and meta-analysis of 90 studies and about 94,000 men found consistent age-related declines in semen volume, motility and normal morphology and a rise in DNA-fragmented sperm, and sperm concentration did not decline consistently; other studies project the declines starting after 35 for some measures and after 40 for others. In a 2003 survey of 2,112 pregnant women, couples where the man was over 45 were 4.6 times as likely as those where he was under 25 to have taken more than a year to conceive, and 12.5 times as likely to have taken more than two years. In 40.5 million US births, fathers aged 45 and over had 14% higher odds of a premature baby than fathers aged 25 to 34, with the absolute risk still low. Age is the one factor on this site you cannot change, which makes it the reason to start on the ones you can.
What changes in semen, and from when
Johnson and colleagues (Ageing Research Reviews, 2015) pooled 90 studies. Semen volume, total motility, progressive motility and the share of normally shaped sperm all fell with age, and the share of sperm with fragmented DNA rose. The meta-analysis did not estimate when the declines begin; other studies project them after 35 for some measures and after 40 for others. Sperm concentration, the number most men look at first, did not fall consistently, which is why an older man's semen analysis can look normal on the headline number while motility and DNA quality have slipped. The rate is gradual; there is no male equivalent of the sharp change women see in their late 30s.
Time to pregnancy
Hassan and Killick (Fertility and Sterility, 2003) surveyed 2,112 pregnant women about how long they had taken to conceive and found that, compared with men under 25, men over 45 were 4.6 times as likely to have taken more than a year and 12.5 times as likely to have taken more than two years. The result held after adjusting for the woman's age, how often couples had sex and lifestyle factors, but it relies on recalled time to pregnancy and only includes couples who did conceive. The direction is consistent with the semen findings, and the size of the effect should be read with those caveats.
The DNA changes
Sperm are made continuously from stem cells that divide throughout life, and every division carries a small chance of a copying error. Kong and colleagues (Nature, 2012) sequenced 78 Icelandic families and found the number of new mutations a child inherits rises by about two for every year of the father's age, doubling roughly every 16.5 years, with the father's age explaining nearly all the variation between families. Most new mutations do nothing. The study is the mechanism behind the modest rises in some childhood conditions that larger studies link to older fathers, and the reason the AUA and ASRM guideline says clinicians should advise couples where the man is 40 or over that there is an increased risk of adverse outcomes for the child. Increased from a low base is the honest framing.
Pregnancy and birth
Khandwala and colleagues (BMJ, 2018) analyzed 40.5 million live births in the United States between 2007 and 2016. Compared with fathers aged 25 to 34, fathers aged 45 and over had 14% higher odds of a premature birth and of a low-birth-weight baby, and mothers whose partners were in the oldest age band had 34% higher odds of gestational diabetes. These are relative increases on small baseline risks: for most couples the absolute change is a percentage point or two. The finding is a reason to know about it, not a reason to panic.
What you can and can't change
You cannot change your age, and the meta-analysis says the decline is gradual. What you can change are the factors that stack on top of it. Smoking, heavy drinking, heat exposure, excess weight and poor sleep each have their own evidence; the main guide ranks them with effect sizes and timeframes. Because sperm are replaced every 74 days or so, a man of 45 who removes those factors is testing a different sample three months later, whatever his age.
When to see a doctor
The NHS advises seeing a GP if you haven't conceived after a year of regular unprotected sex, and sooner if your partner is 36 or over or either of you knows of a fertility problem. The AUA and ASRM guideline uses 12 months when the woman is under 35 and six months when she is 35 or older, and recommends assessing both partners at the same time. A semen analysis is the normal first step for a man; the results guide explains what comes back.
What to do
- If you are 35 or over and planning a family, start the changeable factors now rather than after the first test.
- If you are 40 or over, ask whichever clinician you see about the age-related risks so you have heard them from a doctor rather than a website.
- Get a semen analysis early in the process if you are over 40 or have been trying for six months; it is a normal test, not a last resort.
- Look at motility and morphology, not only concentration. Those are the measures that move with age.
- Track each result across reports. The Motily app keeps them against the WHO limits so the trend is visible.
Related guides
Sources
- Johnson SL, et al. Consistent age-dependent declines in human semen quality: a systematic review and meta-analysis. Ageing Research Reviews, 2015.
- Hassan MA, Killick SR. Effect of male age on fertility: evidence for the decline in male fertility with increasing age. Fertility and Sterility, 2003.
- Kong A, et al. Rate of de novo mutations and the importance of father's age to disease risk. Nature, 2012.
- Khandwala YS, et al. Association of paternal age with perinatal outcomes between 2007 and 2016 in the United States: population based cohort study. BMJ, 2018.
- American Urological Association and ASRM. Diagnosis and treatment of infertility in men: AUA/ASRM guideline. 2020, amended 2024.
- NHS. Infertility.
- Heller CG, Clermont Y. Kinetics of the germinal epithelium in man. Recent Progress in Hormone Research, 1964.
Correction, 8 October 2026. The 2015 meta-analysis did not estimate when declines begin, so the ages of 35 and 40 are now credited to other studies. The Hassan and Killick study is now described as its abstract reports it: a survey of 2,112 pregnant women, compared with men under 25, with the result holding after adjustment. Removed a line that said lifestyle changes reverse within a cycle or two.
