The terms vintage and classic have different meenings as has been discussed. The standard that is assigned to cars is 25 years, but I don't agree that we apply that standard here. IMHO, for Road Bikes, I've noticed that any 20 year old road bike is posted on this forum and no one thinks that's out of line. As an example, most of the Centurion, Dave Scott Ironman bikes are not yet 25 years old, but are readily accepted here. Twenty years coincides with the time when aluminum started to replace steel as the prime tube material and that's another factor that seems to drive us to that time period.
For Mountain bikes, assigning 25 years as the time they became "vintage" is just plain wrong due to the huge advancements in technology for these bikes. I would assign fifteen years as a good vintage time frame for those bikes.
As for classic, I think any bike that is truely different and desirable, not just another "me too" bike might fit that image. I have a 1997 LeMond Maillot Juane that I consider to be classic because if it's desirability, others may disagree.
One thing that is very true, this is a very tolerant forum. I've posted a 93 lugged steel, Celeste Bianchi and no one commented that it was too new to be posted here.