Yes, from 2016 and using a definition of Millenials that went through 1998, so that “Millenials” was the age group including 18 year olds. (Which, if you click through to the cited source, you'll find out that the source has been revised to change.)

This is just restating the long established trend, true across generations, that the youngest-voting age group turns out at the lowest rates (it would be interesting, perhaps, to compare Millenials against other generations voting patterns at the same age to separate out generational from age trends, but this is just misattributing a consistent age trend to the generation that happens to be in an age category at the time of the analysis.)

The first presidential election boomers were eligible to vote in was 1968. Turnout among 18-24 year-olds was over 50%. Similar rates in 1964 and 1972. [1]

[1] https://www.census.gov/prod/2014pubs/p20-573.pdf

> The first presidential election boomers were eligible to vote in was 1968.

1964, actually, but only in Georgia. (Georgia lowered voting age to 18 in 1943, well ahead of the 26th Amendment.)

> Turnout among 18-24 year-olds was over 50%.

There were like four states with a voting age lower than 21, and only one with 18. And it was the height of the Vietnam draft (hence the renewed push for lowering voting age, which had its first big—but nationally unsuccessful—push during WWII.)

So, you've got both external conditions motivating elevated youth vote and stats skewed so that the youngest part of the 18-24 range is underepresented in eligible pool, so it's mostly reflecting the 21-24 subset. But even with all that, and it may not be a big distortion: it's still the worst age group for turnout in those elections, and the movement between and immediately after those years follows general turnout the same way it does through the Gen X periods to the (relative) youth voting surge in 2004-2008, which seems to have subsided in 2012.