Well, technically "vintage" means "of a specific year or era". That drum set that somebody bought new last year would be considered "of a 2011 vintage".
With that said, drummers who are into "vintage" drum gear are into drums from a certain era with specific construction. Once the drums start to be made with thicker shells that project more than resonate (like Ludwig's 6-ply shells, Rogers' XP8 shells, or Slingerland's 5-ply shells...), then those drum enthusiasts are less interested. So, in that sense, "vintage drums" are from those eras, and there's a commonly referred to and understood definition of the word "vintage" by said drum enthusiasts, even though "vintage" means of ANY year/era. It's not snobbery, so to speak, but a specific clarification of a widely-held viewpoint that specific construction methods create a sound like no other drums created since.
The term vintage is not on a sliding scale of sorts, like some people believe. If a kit is 25 years old tomorrow, that doesn't mean that tomorrow it can officially be referred to as vintage. That's just silly...