How many years is a generation?

Let's find out together. And it is not 25.

Sooner or later every genealogist will need to estimate generational length in order to guestimate date of birth or death for a presumed ancestor. It helps if you have an accurate estimate.

The conventional wisdom gives 25 years as a generation but this has no scientific basis and is merely a guess repeated as fact. An estimate of less than 25 years is wildly speculative. The actual answer is about thirty-one years. Here are the reasons why:

• You must include both the father and mother’s age and men tend to be older than their wives.

• In past generations first born babies often did not survive and so did not reproduce and so do not get included in this calculation.

• Until birth control was available in the 1960s women commonly would have children up to about age 40 which raises the average.

• The age of first sexual activity or first childbirth is not the same thing as the mean average of all of the children that live to adulthood and have children of their own.

  A generation is then defined as the average age of both parents at which the average age of their children are born that survive to produce their own children.

And that is why the answer generally migrates to around age 31. I double checked this in my own pedigree going back 382 years through my American and German ancestry. It worked out to 31.83 years per generation.

Grok AI created image of Jerg the Old Stief, born about 1573 in Holzwarth, Wurttemberg. My earliest known ancestor.

In the case of my own family tree going back to Jerg the Old Stief born about 1573 in Germany, 12 generations (from his birth at 1573 to mine in 1955 is 382 years. Counting Jerg and succeeding descendants of his but excluding myself, we have 12 individuals. Three hundred eighty-two divided by 12 yields 31.83 years per generation. If you are dealing with Western European populations, 31 years is a good number. I do not know if this holds true in vastly different cultures found in Asia or Africa.

Thank you to Donn Devine, CG, FNGS, whose answer closely paralleled my findings. You can read his summary by clicking on this link:

How long is a generation? Science provides an answer

Now, here is where your homework comes in! Try this for your own tree. Here are the rules:

• Start with the oldest ancestor in your genealogy and use his or her birthdate as a starting point.

• Count each successive descendant of his or hers down to your birthdate.

• Do not include yourself in the count

• Subtract the oldest ancestor birthdate year from your birthdate year to arrive at the total years.

• Then divide by the number of individuals

• Present your findings in the comment section as follows

  Oldest Ancestor # years # generations result #males vs females

  Jerg the Old Stief 382 12 31.83 7 / 5

Note: Because of the age difference of males vs females, the ratio of males to females will skew the number higher (more males) or lower (more females). In my own example I have 5 females and 7 males in that line, so the result of 31.83 is a bit higher than 31 due to male age bias. If my 12 ancestors in this example were all male, the average age would be about 33 and in the case of females, closer to 29.