By 2025, the state will have 2.6 million people between the ages of 25 and 54, according to the study by the Indiana Business Research Center at Indiana University. That's a 1.7-percent decline from 2005.
In the Indianapolis area, Hamilton, Hancock, Hendricks and Johnson counties will see an increase of at least 10 percent in the age group. Marion County will sustain a loss, but by less than 10 percent.
However, more than one-third of Indiana counties will see their populations of the working group shrink by at least 10 percent. Most are in rural areas, such as Brown and Shelby counties, the study shows.
Indiana isn't the only state facing the decline.
Ohio, Nebraska and five other states will slip at least 10 percent, the study said. At the other extreme, 11 states including California, Texas and Florida, are projected to see their populations of prime working-age residents swell by more than 10 percent.