As the average age of new fathers begins to approach the mid-30’s, many hopeful parents wonder at what point age begins to impact fertility. There are really two questions involved here: Does it become meaningfully harder to have a child as a man’s age increases? And/or are there any implications to the offspring?

Impact On Fertility

Research is starting to show that the answer to both questions is yes. Sperm cells undergo genetic mutations, which means that as men grow older, it's likely that their sperm may contain genetic errors that could impact fertility as well as the health of any future children.

While women are born with a finite number of eggs, which helps to explain the fairly predictable drop off in fertility in a woman's 30s and 40s, men continually produce new sperm, which is why the impact of age on male infertility is more gradual. Only in his mid-40s does a man’s age seem to become a factor. Below is a study looking at the length of time required for couples to conceive, broken down by the man’s age. As you can see, the amount of time required to conceive rises dramatically around age 45.

One of the major issues in this discussion is that older men tend to have older partners, and the impact of a woman’s age is often a driving factor in a couple’s ability to conceive. When the same study looked solely at the age of men who had partners under the age of 25, the impact of male age still looks like it takes most effect beyond the age of 40, around the time of “andropause”, when men begin to produce less testosterone.

When fertility doctors investigated their success rates segmented out by paternal age, isolating out the confounder of maternal age, they noted embryos made by older men were far less likely to advance to day 5 blastocyst stage. A 2013 study looking at 263 donor egg cycles noted that for every 5 year increase in paternal age there was a 26% reduction of likelihood of a live birth.

Impact to Offspring

At what age does it become dangerous for a man to reproduce on account of his offspring’s health? The truth is, that’s still hard to say. It's still uncommon for men to father children after, say, the age of 50, and since the downstream conditions linked to genetic anomalies (for instance, autism) are still relatively rare, there currently isn't enough data to make any inferences.

Autism Spectrum Disorder

One large population study out of Israel noted that children fathered by men over the age of 40 were five times more likely to develop Autism Spectrum Disorder. While that relative risk is high, ASD is uncommon and so the absolute level of risk equated to about 3 in 1,000 babies.

Bipolar Disorder

One Swedish study of 14,000 people found that children born to men over the age of 50 had a 37% higher likelihood of developing bipolar disorder, even after correcting for variables like the mother’s age (which had a slight effect) and socio-economic status. Roughly 1 in 100 Americans are diagnosed with bipolar disorder, and so the absolute raised level of risk is still likely well below 2%.

Schizophrenia

The seminal study investigating advanced paternal age and schizophrenia (acquired by age 34) comes out of Israel. This study showed a steady, but mild, increase in progressive risk that ultimately jumped up when men fathered children after the age of 50.

Emotional Well-Being Upon Parent Loss

Another factor to consider is the burden placed on a child to care for their older parents. As more than one researcher has pointed out, the emotional pressures of elder care can be daunting for a young man or woman trying to establish themselves in the world.

Macabre as it may be, the reality is that when a man fathers a child after the age of 55, there is a 20% chance that father will die before the child turns 15. By all acounts, children who suffer the loss of a parent from natural causes are more prone to depression and to need mental health care.

Long Tail Diseases

Ultimately, paternal age is believed to have an impact on a number of conditions, the vast majority of which are extremely uncommon. A team from Denmark took a close look at the literature, and their own population, and tried to ascribe the impact paternal age has on conditions ranging from the prevalent to the incredibly rare.