The notion of "generations" as way to describe people born during a particular period of time is a relatively modern one so we're unlikely to see primary texts that reflect the modern framing. Robert Wohl's 1979 book The Generation of 1914 is probably the most in-depth look at the concept and the book is a good read for getting a sense of how men born at different times were positioned and how men in various decades sought to make sense of changes over time.

That said, in America up until the mid-1800's, age was a vague construct. The most comprehensive book on the topic, How Old Are You: Age Consciousness in American Culture by Howard Chudacoff, traces a number of themes to explore how we thought about age, including language. I get into more detail about how we fell into a stricter association between times around the sun and a number here and here, but generally speaking, people didn't have an exact sense of age because they didn't need it. From a previous answer:

Prior to the 1850's or so in America, biological age was a loose frame for stages of life. "Youth" was used to refer to people who were seven or thirty. "Boy", likewise, had different meanings, depending on how it was used and who it was used by and about and "spinster" was used to describe unmarried women in their early 20's in certain contexts.

The notion of separation by age was also shaped by race, gender, and class. We can see these vague categories in etiquette books of the early and mid-1800s which described a younger class of females and distinguished young ladies from aged ladies but there were no set age boundaries. In effect, a white wealthy grandmother and her granddaughter would be greeted and treated differently explicitly because of their age. However, an enslaved child would be expected to work alongside parents and grandparents. In some parts of the country, Indigenous children were kidnapped from their families by white colonizers with the explicit goal of re-educating them to impacted future generations of a given community. Likewise, some Indigenous communities sent their children away to receive an American education with the hope they would return to help older generations deal with the encroachment of Europeans on their land. Finally, "Boy" was used by white adults and children to address and describe Black men until well into the modern era, even when the Black man was older than the speaker.

Back to schooling. During the rise of the common schools and the creation of teaching tax-payer systems, teachers would typically be only a few years older than the students. Occasionally, they were younger than the oldest students. However, students saw her as an authority figure not because she was older, but because she was the teacher. (As an aside, those doing the teaching were mostly white, mostly unmarried young women. Those running schools, though, were virtually all white men, typically a decade or so older than the women teachers as they routinely attended college before becoming a schoolman. Older unmarried teachers even had their own moniker: schoolmarm. Like spinster, the term was used for women as young as 20 and as old as 60.) As the system expanded, cities and states began to establish laws and policies around when a child could attend school. Some did it sooner - before the Civil War - while others wouldn't establish ages of minimum enrollment until well into 20th century.

Despite the lack of specific age markers, students routinely saw themselves in conflict with the adults running school. Young men at Harvard (where the average age in 1780 was 10 1/2) routinely complained about tutors. Students in one room schoolhouses were prone to organizing collective action against particularly cruel or unpopular tutors or teachers and young people often wrote letters or diary entries struggling with the expectations of the adults around them. In a modern sense, we can see these as intergenerational conflicts, but it was also the perpetual tension between those with and those without access to power and how those with power saw those without and how they dealt with changing social norms. It's the same tension that lead to Aristotle complaining about students in that answer you linked to.

America started to get more precise about age in the late-1800s and we see that in a variety of ways: Congress passed laws requiring men to be of a certain age in order to serve in the military. The idea of "adolescence" appeared in the literature and adults began to put money and effort into articulating what young people who were not yet adults, but no longer children, should do. However, there was no widespread social expectation from adults on young people to finish 13 years of education. So, this meant it wasn't uncommon for a teenager to be simultaneously a child and an adult. They were developmentally still growing, but out of school and working. It would take until after World War II for the lines to become sharper and clearer. (A longer look at why 18 became the magic number to indicate adult here.)

And this leads us back to your question: explicit intergenerational tensions, to a certain extent, can be traced back to the children born between 1946 and 1964. While conflict between adults and young people for sure existed prior to the arrival of Baby Boomers, they were the first generation of Americans, regardless of class, expected to follow a relatively set path: 12 or 13 years of public education, college or trade school or marriage or job, then family. This meant that language began to emerge to describe those who didn't follow the path. The parents of and adults around Baby Boomers began to call young people who left high school dropouts to indicate they deviated from the path and young people created a culture around the label. Feminists coined the title Ms. to remove the indication of a woman's marital status. Finally, the notion of cliques in high schools emerged along with the idea there were different names for young people who lived within the rules set up by adults (i.e. teachers' pets, brown nosers, etc.) and those who didn't (I.e. greasers, juvenile delinquents, etc). Which isn't to say they were the first generation to develop new language to define themselves and the world around them, but rather, they were the first generation with a specific start date and an organizing center.

So, basically, the American generation with the reputation for complaining about "kids these days" were the original "kids these days."