In many parts of the Northeast and Midwest, population growth is slowing at an unprecedented rate as people are getting older, women are having fewer children, and more people are moving out than in — and that signals big economic trouble ahead. The population of prime working-age adults, ages 25 to 54, will decline in 16 states, most of which are in the Northeast and Midwest, from 2010 to 2040, according to a Stateline analysis of projections released by the University of Virginia’s Demographics Research Group in the Weldon Cooper Center for Public Service. Maine, Vermont and West Virginia will see their working-age populations drop more than 10 percent. Connecticut, Illinois, Michigan, New Hampshire, Ohio, Pennsylvania, Rhode Island and Wisconsin will see theirs fall more than 5 percent. State officials in Illinois, Maine and Pennsylvania say they’re already seeing holes in the workforce as baby boomers near retirement. To stem the trend, they are trying to ensure that young people are educated and trained for existing industries, and to help bolster the sectors that show promise of expanding.