According to the standards and recommended practices set by the International Civil Aviation Organisation the air traffic controllers must meet a number of specific requirements with regard to age, knowledge, experience and medical fitness. Candidates must be at least 21 years of age (before being issued a licence). Minimum experience requirements and standards of medical fitness are detailed in ICAO Annex 1. All student controllers must demonstrate an appropriate level of knowledge in at least the following subjects: air law, air traffic control equipment, general knowledge of the principles of flight, human performance and limitations, language, meteorology, navigation and operational procedures.