Generations
If you have ever used a term like "Boomer" to describe a generational cohort in the United States, you've articulating a generational theory first formalized in the book Generations by William Strauss and Neil Howe.
The book articulates a series of crisis points that occur on a roughly eighty year schedule. Later books such as The Fourth Turning Is Here (2023) provide updates.
While the core premise is hotly debated, the use of generations as markers continues to be popular as it provides such a clean model for describing both shared experiences and intergenerational conflict.
Even if you don't agree with the predictions in Generations, I found it to be a highly enjoyable, extremely well cited tour through the cultural history of the United States.