Generations is a very… imprecise… sociological concept. People just sorta look at a rough cohort of ages and say “yeah, you all are a generation.” Start and end points (as well as labels) just sorta coalesce out of the ether.
Nevertheless, they have their use and certainly are cemented in the popular imagination. I’ve been doing some reading on generations and really appreciated this diagram and article by Pew Research. But I wish it went a little earlier and a little later. So I made my owner version, *heavily* influenced by the Pew version.