This map shows, in blue areas, where the counties are that contain the youngest Americans. My theory is that most of those blue areas are college towns, Indian reservations, areas with high percentage of farm workers, and certainly Mormon areas in Utah. The original map is here.
Below is a close up of the Pacific and Inland Northwest. Whitman County – home to Washington State University, is the youngest county in the entire region. Latah County, Idaho, home to the University of Idaho, is adjacent to the east. The prime farming areas of central Washington also contains many young people. I though that the counties that contain Seattle and Portland would skew bluer, but do not. The mountains of eastern Washington, northern Idaho, and western Montana have a comparatively older population.