A certain fraction (cause it's never everybody) experienced their periode of being X years old during a certain time and a certain place. And they consider this "the norm". When they look back, and see others being in the same periode of their own life that overlaps with the periode they already had and developed a "norm" for, and they see this new generation has moved away from the "norm"... then they run the risk of marginalising the new generations experience. It's not like theirs, so they don't understand it, so the new generation must be doing something wrong.
Ussually, people that look back, also only think of a certain number of experiences. Because recalling everything is a pretty hard thing to do for a human being. So "the norm" is ussually a verry subjective romantisation of what they actually experienced in their own time. Because memories fade, memories change and sometimes memories are just constructed. It's pretty hard for the youth to battle against romantic recollections of the past.
At least thats my idea... this has been going on for centuries. You find texts in ancient Rome of people looking back, and talking about the good old days.
