The question of when a man stops being fertile is often misunderstood. Unlike women, who experience a definitive cessation of fertility with menopause, men do not have a biological “finish line” where the ability to father a child abruptly ends. The male reproductive system continues to produce sperm throughout life, but this continuous process masks a gradual, age-related deterioration in sperm quality and function. This decline affects both the probability of conception and the health of potential offspring, resulting in a significant reduction in reproductive capacity.
Fertility Decline Versus Complete Cessation
Men continue sperm production (spermatogenesis) well into their later years, meaning they are theoretically fertile for life. However, the efficiency and success rate of conception begin to decrease noticeably starting around 35 to 40 years old. This reduction is a progressive decline in functional capacity, not a sudden drop-off. Studies show that a man over 45 may take five times longer to achieve a pregnancy compared to a man under 25. The issue is not complete inability to produce sperm, but a decrease in the likelihood of a healthy conception. The overall viability of sperm is compromised with age, resulting in subfertility and a higher risk of failed conceptions.
Age-Related Changes in Sperm Quality
The primary reason for the decline in male fertility is the age-related deterioration of the sperm itself. As a man ages, the physical characteristics of his sperm worsen, including a reduction in semen volume and a decrease in sperm motility (the ability to swim effectively). The morphology, or shape of the sperm, also tends to become more abnormal over time, limiting the sperm’s capacity to fertilize an egg.
A more concerning change is the increase in damage to the sperm’s genetic material. Older sperm accumulates more DNA fragmentation, which involves small breaks in the genetic code. This fragmentation can interfere with embryo development and is associated with an increased risk of miscarriage and lower success rates in fertility treatments. Furthermore, the stem cells that produce sperm undergo more replication cycles over a lifetime, increasing the chance of new genetic mutations, called de novo or germline mutations, accumulating.
The Role of Hormonal Decline
The systemic hormonal environment changes with age, indirectly influencing fertility. Testosterone, the primary male sex hormone, begins a gradual decline starting around age 30, with levels decreasing by about one percent per year.
A drop in testosterone can affect reproductive function by reducing sexual desire and causing erectile dysfunction, which makes conception more difficult. While sperm production is mainly stimulated by other hormones, the decline in testosterone and related changes in the testicular environment can indirectly slow the rate and efficiency of spermatogenesis. Age-related changes in the testes, such as a decrease in the function of Leydig cells, contribute to this less optimal environment for creating high-quality sperm.
Late Paternal Age and Offspring Health
Delayed fatherhood, typically defined as paternal age 45 or older, is associated with a greater risk of adverse outcomes for the partner and the child. The increase in sperm DNA damage and de novo mutations is the biological mechanism behind these risks.
Children born to older fathers have been linked to a slightly higher incidence of certain non-chromosomal conditions. These include neuropsychiatric disorders, such as autism spectrum disorder and schizophrenia. There is also an increased risk for specific single-gene disorders, such as achondroplasia (a form of dwarfism), which are tied to the accumulation of new genetic mutations. Additionally, late paternal age has been associated with higher rates of premature birth, low birth weight, and an elevated risk of the pregnant partner developing gestational diabetes.