After extensive research, we propose 1922 as the year in which air traffic control began in earnest.

The first internationally agreed rules, drafted in 1919 by the Commission Internationale de Navigation Aérienne (CINA), were ratified by 10 countries in 1922.
The first mid-air collision between two commercial airliners over Normandy (France) in April 1922 further encouraged the UK, France, Belgium and The Netherlands to implement guidelines for keeping aircraft separated, thus forming the basis for air traffic control as a service to avoid collisions.