If you all read the Pew research used in this article and cited in its second paragraph, most of your questions will be answered.
For example, the millennial age group is defined as born 1981-1996. By 2014, nearly all of the millennials were of voting age.
Yadda yadda yadda.
That's Pew's range for defining the group. The problem is that the oldest would have almost been in high school before the internet was widely available, and it really doesn't fit the idea of Millennials being the tech savvy young generation.
The other is they make the Boomer generation 20 years, but GenX and Millennials just 15 years, a completely arbitrary designation.
In reality, Millennials should be from 1986 - 2005, which far more fits their life experiences that you think of for that generation.