The study also says that by 2040, retirees will outnumber people ages 25 to 54 - workers who are critical for building economies.
The study was published recently in Indiana Business Review, an organ of the Indiana Business Research Center. The center is an arm of IU's Kelley School of Business.
Demographers Matt Kinghorn and Rachel Justis cautioned that the projections are based on recent trends in births, deaths and migration, and make no attempt to anticipate economic or environmental conditions.
The authors also remind readers of the study that other states are experiencing similar shifts.
If the predictions pan out, implications for everything from hospitals to road construction will be broad.
Indiana's overall population will rise 15 percent, to 7.2 million, by 2040, the study says.
But nearly a third of counties will lose population.
In the meantime, the Indianapolis area will receive more than half the state's population growth. Only the northeastern counties of Elkhart and LaGrange will experience similar growth; both have extensive Amish and Mennonite concentrations, which have high fertility rates.
Retirements of baby-boomers will place a heavy burden on younger people in the work force.
One of five Hoosiers will be of retirement age by 2040, about twice the current level. In the meantime, the number of workers in the key earning years of 25 to 54 actually will decrease.
"Will Indiana have the labor force to grow, or even maintain, its current level of economic activity?" the study asked.
Most of Indianapolis' suburban counties won't feel a lot of pain compared to the rest of the state. Workers especially will flock to Hamilton and Hendricks counties.
Morgan and Shelby counties on the southern edge of the metro area are exceptions. Those counties will see only slight increases in overall population and will lose people of working age.