In July 2013, the low-cost Irish airline, Ryanair, astounded the world by hiring a 19-year-old pilot on a Boeing 737. A rare case, but legal.
Firstly, what has to be borne in mind is that all airline pilots begin their careers as co-pilots - never as captain. To be a co-pilot on a commercial aircraft there is a minimum age requirement; the law is very specific on the subject.
The International Civil Aviation Organization (ICAO) has set its member states’ co-pilot minimum age at 18. To be a captain, you must be at least 21 years old and have completed a minimum of 1,500 flight hours. These standards may be tightened, or relaxed, for each of the 191 countries, if they so request.
The United States is an example. For American airlines, the US Federal Aviation Administration (FAA) imposes a minimum age of 21 years and 1,500 flight hours for commercial aircraft co-pilots. To obtain the post of captain, applicants must be at least 23 years of age and undergo an additional forty hours’ training.