There are more than 150,000 registered nurse and licensed practical nurse licensees in Massachusetts, but that number has increased by only 15 percent or so in the past 25 years, despite a growing population that’s also getting older. According to the American Association of Colleges of Nursing, the U.S. is projected to experience a shortage of registered nurses (RNs) that is expected to intensify as Baby Boomers age and the need for health care grows.