Christopher Michel/Wikimedia

Soon, Millennials, we will be overtaken. The trend pieces have already started defining the upstarts, the next generation. As Millennials replaced Gen X, and Gen X replaced the Baby Boomers, and Baby Boomers replaced the Greatest Generation, we, too, will be replaced by the Youth of Today.

This is not, however, some ancient aging curse that's afflicted humanity since the beginning of history. Societal generations are a relatively modern idea, hit upon by 19th century European intellectuals and refined in the beginning of the 20th century.

Back in 1979, the historian Robert Wohl (now an emeritus professor at UCLA) took a close look at "the phenomenon of generational thinking" in The Generation of 1914. Before the 19th century, generations were thought of as (generally male) biological relationships within families—grandfathers, sons, grandchildren and so forth. But in the 1800s, that started to change, Wohl wrote:

One can trace its progress in dictionaries. During the early 19th century the term "generation" was used primarily to signify either the relationship between fathers and their sons or contemporaneity. The French lexicographer Emile Littré defined a generation in 1863 as "all men living more or less in the same time." In the second half of the nineteenth century the term was employed increasingly to connote coevals, and especially to evoke the dichotomy between the older generation and "youth."

The roots of this idea came from the work of French and German philosophers who were, the socialist Karl Mannheim wrote in 1927, "anxious to find a general law to express the rhythm of historical development, based on the biological law of the limited life-times of man."

But the idea of societal generations became particularly compelling to young intellectuals living in European cities in the early 20th century.
It helped them, first of all, explain why their own era-defining creative and philosophical pursuits were important and special—they, the rising generation, were pushing society forward! Each generation had a shot, and this was theirs. Here's the Futurists, for instance:

The oldest among us are not yet 30 years old: we have therefore at least 10 years to accomplish our task. When we are 40 let younger and stronger men than we throw us in the waste paper basket like useless manuscripts!

But it also created a framework in which to explain some of the sweeping changes happening, anyway: More people had the luxury of a life stage between childhood and adulthood; those young people had their own distinct politics; technology and war were outmoding whatever wisdom their parents might have offered. Mannheim put it like this: "Youth experiencing the same concrete historical problems may be said to be part of the same actual generation."

Is this actually a helpful way of grouping people? It gets more complicated, as Mannheim pointed out, when you start considering that people do not react to their particular historical conundrums as a monolithic group. This is often where trend pieces get into trouble, when they start attributing one possible reaction to the group as whole. (“Millennials are lazy, entitled narcissists”; “In contrast to the view that millennials are lazy and entitled, millennials are extremely optimistic...”)

To the extent that a “generation” is a helpful way of organizing thoughts about larger group of people, maybe it's a simple as this: These are the people who share your problems. Some of them might have come up with solutions that work for you, too. But some of them might just happen to have been born “more or less” when you were.