But there are a few requirements applicants must meet before they fill out the initial paperwork. Applicants must be U.S. citizens under 31 years of age (controllers still face a mandatory retirement at age 56). Applicants must pass medical, security and pre-employment tests and speak English clearly enough to be understood on the radio. Trainees must have at least three years of responsible work prior to applying and be willing to relocate anywhere the agency requires.