The levels of guard include: Novice, Intermediate, A Class, Open Class, and World Class. There are also Cadet and Junior guards for younger youth and Senior Guards for those who are older. These levels deal with a certain set of criteria but by and large it all has to do with ages. .
Typically A Class goes from about 13 years old to 21 years old. You may not turn 22 by the time WGI World Championships are held.
Senior groups have an age minimum but no maximum. That age is 24.