"Historians have a habit of measuring time in generations. Broadly speaking,
a generation is the time it takes a new-born infant to become a reasonably
mature adult. While there is no fixed period for such an evolution, a common
presumption is that it takes about twenty years. A generation matters because in that time, the combined impact of education and experience can produce a human with distinctly different ideas, expectations and understanding of the world. Perhaps that is why it is meaningful for us to refer to “Gen-Xers” and “Generation Y”, though our alphabet is running out of space."
Professor Desmond Morton