It depends on who you ask.

Some people go strictly by a 20, 25 or whatever year age. But that age isn't codified anywhere- and it's different with different organizations and different insurance companies and government agencies.

If someone were to ask me what I think, not that anyone cares, there's a difference between an "old" bike and a "vintage" bike- and generally- we're talking about bikes with level top tubes with non-oversized tubing made before 1995. I don't use 1995 as a random "30 year" or whatever date- but Suntour went out of business in 1994.

Here's the longer, more stupid version of my theory... you probably know the word "vintage" is derived from the term meaning "wine harvest." The idea is that there are good years for wine and not good years for wine- not necessarily how old the wine is. So if the soil is right and there's the perfect balance of sun and rain and the temperature produces the best wine ever for the 1969 wine season- making the 1969 Chateau de GoldenBoy one of the finest vintages ever. But 5 years before- if there was a flood and dumped a load of sewerage on the vineyards and it was cold and crappy- making the 1964 one of the worst years... even though it's 5 years older, it's a significantly worse wine. I think the idea is more the properties of the "harvest" that make a wine, bike, car... etc desirable- not the age.

To muddle the waters a bit, this is "CLASSIC and Vintage," hedging the whole thing to bikes that are made in that "classic" sort of idiom- irrespective of when they were made.

On top of all that... newer bikes are generally more appreciated by younger people and "vintage" bikes are generally more appreciated by "vintage" people- and as these people drop out of the hobby- the older stuff becomes less appreciated. So while old people might roll their eyes at the idea of a 2005 Trek Madone being considered "vintage," without younger people into the "vintage" hobby and forums- it's going to get awfully sparse in here.