In recent weeks the U.S. Census Bureau released more detailed demographic profiles obtained during the 2010 count. Unlike redistricting data, which was released earlier this spring, the demographic profiles break out the population by specific age categories.
This map shows more than 3,100 U.S. counties (or county equivalents, like parishes) with the percentage of population that is younger than age five. As you can see, there’s wide divergence across the country, but some western states — Utah, Arizona, Texas, the Dakotas and Alaska — have several counties with a large percentage of young residents:
I suspect it’s because these states, for a variety of factors, including religion, just have higher birth rates. Here’s the view by state: