I think it's based on the trope that teenagers rebel against their parents' beliefs, mixed with the idea of generational progress. So each generation of teenage

I think it's based on the trope that teenagers rebel against their parents' beliefs, mixed with the idea of generational progress. So each generation of teenagers rebels against their parents' ideas that they believe are "outdated", and then supposedly grow up retaining this new framework of belief. So what was cool becomes uncool, what was progressive becomes conservative, etc etc. I think it's mostly nonsense though. I think the underlying grouping that generation is a proxy for, is age group. 18-25 year olds behave differently from 25-35 or 35-50 (with some fuzziness at the edges). In other words it's loose grouping based on age/maturity.