Re: How old an instrument can be called "vintage"?
I'd say anything older that keeps its value or is of significant quality and is available in limited numbers.
I wouldn't consider my 2001 Les Paul vintage, though it is a very good guitar and come from a limited run.
Nor would I consider my early 70's acoustic vintage, since it is, quite frankly, not that great a guitar, even though I bet there aren't that many of them left (possibly due to not being anything that remarkable to begin with)
I would however consider my 89 Charvel one. Even though it may not have passed the 30 mark yet, it fulfills all of the above requirements.