The Federal Aviation Administration. The Social Security Administration. The National Science Foundation. The Treasury Department. All could lose as much as a quarter of their employees by 2012, mostly because of retirements. They are not alone. Across the government, about a third of full-time employees will retire in the next five years, according to estimates prepared by the Office of Personnel Management. The turnover could be even higher in the ranks of federal executives and supervisors. From the start of the Bush administration, agencies have been preparing for the churning that will be caused by the baby boom retirement wave....