The age structure can be a really important part of the economic vitality of an area. A diverse age mix encourages a mix of good and services offered, and can really do so over time. Also, as North Dakota grapples with attracting population in general, understanding the age profile choosing to remain here can be important information considered by policy authorities.
The counties with median age less than the state median age are, to some extent, scattered across the state. The oil patch, Native American reservation counties, counties with large universities all seem likely counties to have high populations of younger people. It is interesting to see the scattered pattern of the older populations as well.
Overall this is a pretty good situation for the state with one issue. The oil patch worker population is overwhelmingly younger, but is less likely to represent permanent additions to the state population. They come for jobs, specific jobs, and often do not have other factors keeping them here. If these jobs become available elsewhere, or if the jobs here simply disappear there is less to keep this younger population here. Those are issues we need to address and confront., before we can deal with them in a policy approach, if that is even possible.