Birthyear IS relevant, because the same outside forces affect each person of that birthyear throughout their lives. Now, different outcomes obviously come from this, but you can't say that trends do not happen with people born at the same time. For instance, I could tell you that my first concert was Foghat/Pat Travers. That has no significance to most anyone who is over 50 or under 30. But someone who is 40 would have a definite reaction - either "Kewl!" or "Yuck!", but someone that age would have the same point of reference.
So for trending purposes, this stuff does matter.
yes, but it has significance to me and i am a late boomer and you are a Gen Xer (my first concert was Humble Pie, does that have significance to you?) so what does that prove? i was in elementary school when Woodstock was going on, what did i know about it? but as a boomer, that was supposedly MY TIME. it is just NOT accurate.