Gen X here. I've got an average 123 WPM on typeracer, which puts me in the 99,8th percentile.
I started looking at the screen instead of the keyboard early on. There were touch typing classes as an option around 8th grade, I think, but it was literally just having a map of which fingers go where and typing text focusing on using the right fingers. I didn't take one, but I think I'm using the right fingers for 80% of the keys. I'm moving my hands back and forth a bit to let my dominant fingers do the work.
I started playing MUDs in 1997 at age 13, and building up that muscle memory for every combination of two- or three letter commands probably did more than I'd care to admit. I still miss the responsiveness of a proper DOS prompt, or Linux tty.
Millennials were born in the late 80s and through the 90s. 97/8 is the rough cutoff. Most of them are in their 30s now and the oldest are in their early 40s.