I believe that when the F.I.M. met in 2002 they made the following decisions:
"Classes for Trial World Championship and FIM Junior Cup
As from 1.1.2004: 125cc class, 16-18 years old with driving licence, 4 stroke up to 125cc, minimum weight 70 kg. Open Class, over 18 years old with driving licence, 2 stoke up to 250cc and 4 stroke up to 450cc, minimum weight 75 kg.
Open class as from 1.1.2005: only 4-stroke up to 450 cc, minimum weight 75 kg."
I have not seen any reversal of the decision but that does not mean it has not occured.