No, there is no age limit for the training other than you must be legal enough to play paintball (legally)
No, you don't have to take the class to ref the GPL or if you refed in the past. However, the GPL is ONLY going to take best of best for their referee staff.
The GPL with PAINTBALLid.com is organizing a Referee database and a Refferee ranking system from Level 1 to Level 5 with Level 5 being the best, most experience and highest paid Referee.
In order to move up from Level 1 to Level 5 there will be some mandatory training along with "X" amount of events worked in order to move up. This is not only for GPL events but for all other fields and events that would like to participate in using the referee database to inssure they have the best and most experienced referee's in order to make their events better for everyone around.
There is no other qulifications. Anyone can sign up. We are going to train you.