The term biological clock was coined in the 1970s to describe the pressure women felt in their 30s to have children before they lost the ability to. A woman's fertility declines starting after 30 and goes to 0 by 45. Men's fertility declines slowly and still is ok past 40. So men don't have to worry too much right? Wrong. Fertility is not the only quantity with an expiration date. Humans expire too and if you wait too long to have children, your parents are not going to be alive to know their grandchildren and vice versa. This is another biological clock.

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This is not theoretical. My maternal grandfather was 80 when I was born and - while he lived to the ripe old age of 88 - he probably saw me at most a dozen times and I was too young to form any memories of him. He was lucky, one of my aunts prematurely died in her 60s a year before her daughter got married and gave birth to a granddaughter. Not knowing my grandparents well is one of my life regrets. Nobody has control over when they were born, however you do have control over when you become a parent.

Let's look at actuarial data to see how much time is left. Below is an actuarial table from the Social Security Administration showing the expected remaining years of life for people of different ages. Consider a person who was born when their parents were 30 and who has a child at age 30. Assuming their parents are still alive, we can expect on average the grandmother will live til age 84 when the grandchild is age 24 and the grandfather will live til age 81 when the grandchild is age 21. That is not too bad. A man who becomes a grandfather at 80 can only expect to live to 88 when the grandchild is 8.

Having a pulse is not much use if there isn't a mind to go with it. Dementia can precede death by years. 5% of adults 65-74 have Alzheimer's and the rates rise to 14% for ages 75-84 and 36% past 85.

Source: Alzheimer's Association 2026 Alzheimer's Disease Facts and Figures

I recently hiked the Grand Canyon with my 70 year old father. Seeing him hike 9 miles with ~4000 feet of elevation gain in a day made me less pessimistic about aging. Still, there are only so many more of these trips left. This idea is captured by the concept of healthspan which is how long a person can live an active life. According to the World Health Organization, we can expect a hypothetical 60 years old grandfather and grandmother to have 15 and 16.5 years of healthy life left before becoming too feeble to do much. The 80 year old grandparents likely would have exhausted their health span. In conclusion, time is ticking so go out there and seize the day. Carpe Diem!