An explanation of age categories:

Currently, there are three broad age spectrum’s in road running: Juniors (under 20), Seniors (20 to 39) and Masters (40 plus). The old terminology of Sub-Vets, Veteran, Master, Grandmaster and Great Grandmaster are still commonly used in South African running clubs, as per the table below, however these have officially been dropped by the IAAF because with the growing numbers of juniors and masters category runners in the sport this categorizing has become an issue.

There is also a difference in the way ages are calculated. For Juniors and Seniors your category is defined by the YEAR in which you were born. Eg. if you turn 20 on the 10th January 2018 or the 28th December 2018, you will be regarded as a Senior the entire 2018. Whereas with Masters categories your age category is based on your age ON THE DAY OF A RACE. Eg. if you enter Comrades Marathon for 2018 and your 40th birthday is on the 9th June you will be in the Senior category on race day on 10 June. However if your 40th birthday is on the 10th June you run as a proud Master.