Hi
I agree with Graeme M and even "The Grand Poobah of Firkoland" , and sooner or later there will need to be a division for a variety of reasons , track suitabilty will be the first one , for me the cut off will be pre 75 or pre 78 because of the suspension limitations and track conditions ,.
dont let this be an us and them argument , we will all ride vintage bikes, hopefully just on more suitable tracks , I think its safe to say Evos pre85,pre90 onward have the suspension capable of running on any motocross track in Aust, maybe pre 78 as well ,