Generally speaking, in AHRMA a vintage bike is 1974 or older. There are exceptions. The AHRMA also has a relativly new Post Vintage series. For Post Vintage you are looking at bikes as new as early 80s. But it depends on how the bike is equipped. For instance a 1992 CZ 400 would be legal for the the GP class. The year is unimportant as long as the bike meets the class formula. You can check their online rule book to see where your bike fits in. http://www.ahrma.org/rulebook/
There are a lot of vintage groups and clubs that don't follow the AHRMA guidelines. So it sort of depends on where you are and what type of racing you want to do.
So what kind of a bike have you got?
'00 Honda XR400
'81 Bultaco Sherpa T
'73 CZ 380
'70 Bultaco Sherpa T WesBaca.com