I guess the voter turnout among the 18-29 voting group age should reach at least between 55 % and 60 % in 2020 since all millenials will be eligible to vote, the same for the first post-millenials (Generation Z) born before the end of 2002.
Not really. I think you are grouping together voters that shouldn't be grouped together. The Millennials who will have higher turnout will be the older ones, who will fill most/all of the 30-39 age block. Those are the higher turnout ones. The younger ones will have lower turnout.
Also some non-Millennials will be in 18-29 group, and their turnout will likely be as bad as it has been historically. The 18-29 year olds should have turnout similar to past elections, although I suppose it could go higher if the election has really high turnout and maybe some 2008-like feel to it. But it's not going to go to 60% due to Millennials. The higher turnout due to aging will represent itself as it always does - with older people.
TL;DR nothing is going to change with 18-29 year olds, at least not in any way that hasn't affected past people who were 18-29 years old at some point.
I think 2020 will have a 2008-like feel.
Every generation is different, and is shaped by widely differing events. Reported turnout among 18-20 year olds dropped 10.3% from 1972 to 1976, even though overall reported turnout dropped only 3.8%: the Vietnam War and draft were no longer an issue.
My generation (early Gen-X) grew up in the cynical, politically apathetic 1970s. In addition to being the first generation in 30 years not to start out as liberals, we were also relatively politically apathetic. Tell most 21-year-olds in 1987 that political involvement was both personally satisfying and a way to a better future, and they probably would have laughed.
Today's Millennials and Gen-Z are different. They know much more than we did about how broken politics is, and they honestly believe (correctly, I think) they can make a difference. I think today's Americans younger than 30 will be much more involved in the process than we were in the late 1980s, and it will show.
