Here is one paragraph:
Here is this change in a single number: The interstate migration rate has fallen 51 percent below its 1948–1971 average, and that number has been falling steadily since the mid-1980s. Or, if we look at the rate of moving between counties within a state, it fell 31 percent. The rate of moving within a county fell 38 percent. Those are pretty steep drops for a country that has not changed its fundamental economic or political systems. You might think that information technology (IT) would make it easier to find a job on the other side of the country, and maybe it has, but that has not been the dominant effect. If anything, Americans have used the dynamism of IT to help ourselves stay put, not to move around.
Here is the rest of the piece. It is not mainly about age demographics, and we have in fact outsourced much of our geographic mobility to immigrants.