For larger events such as Otago Champs, South Island Champs and Nationals you enter according to your gender, age and ability.
e.g. M21E, W40B, M12A
An age class normally has three parts;
Traditionally, beginners would start with C grade courses, then once they start to win that grade, move up to B grade. B Grade winners would move up to A grade, etc.
Note that you can be any age to run Elite class, you don't just have to be between 21 and 40 years old - but you have to be able to handle the distance and technical difficulty.