A yearly Government Accountability Office report on NASA’s major projects outlined on Tuesday several challenges that the space agency faces, including cost growth and launch schedule delays. But the report also addressed a more complex topic: NASA’s aging workforce.

More than half of NASA’s workforce is 50 years old and over, marking a seven percent increase over the past five years, GAO said in its tenth annual report. 56 percent of NASA employees fall into that category. About 21 percent of NASA’s workforce (made up of 17,063 people) was eligible for retirement at the beginning of 2018, and on top of that, 23 percent will be qualified to retire in the next five years. Officials pointed to NASA’s low employee turnover rate (about four percent annually) as one reason for the trend.