"As baby boomers age, the nation has substantially aged as well. In 1970, the median age in the US was 28.1. In 2016, it was 37.9."
The second map doesn't show "movement" of individuals, but the changing age distributions are equivalent to movement of populations.
Lots of implications, which I don't need to elaborate on.
Note the maps at the source are interactive (zoomable, and if you hover over your county the raw data pop up).