OK, here's my 2 cents...
The term vintage comes from wine making and refers to the year the wine was made and bottled. You might say the vintage of a given bottle of wine is 1993. Obviously people also use the term for other things with the implication that that thing is both old and good (like wine?).
So a literal definition of "vintage" requires that you be talking about wine. A less literal definition means that *something* was made *sometime* in the past. The most common usage today is that a thing is relatively old and probably better than something made today. How old does it have to be to meet this last definition? It changes. When I first got interested in "vintage" guitars and amps a Fender guitar had to be pre-CBS and an amp had to be blackface. Then guitars had to be pre-1970 and amps could be silverface but had to be pre-pull boost. Now people are paying crazy money for "vintage" big headstock '70s guitars and pretty much anything silverface is considered old enouh and good enough to be considered valuable.
-Scott