Actually the claim that 1,000,000 baby boomers moved to within 5 miles of downtown in the 50 largest cities between 2000 and 2010 is patently false. Census Bureau data shows that there was a 66,000 loss in population within 5 miles of the city halls of the 51 metropolitan areas over 1,000,000 population. See: “Flocking Elsewhere: The Downtown Growth Story,” http://www.newgeography.com/content/003108-flocking-elsewhere-the-downtown-growth-story.
There is no way a net 1,000,000 baby boomers could have moved so close to downtown, and the data shows it. The actual census data on baby boomers shows a loss of more than 1 million boomers inside the core cities, which largely include this 5 mile radius.
The authoritative source for such data is the United States Census Bureau (which is the source of the data referenced here).
Also see: Similar Wall Street Journal Error