I think that 25 years is a good year of thumb. But the more complex answer has to do with when a particular type was introduced. If we are talking about Roadsters something made in the 1970's barely qualifies. But if we are talking about Mountain Bikes if we used the same date there wouldn't BE any of them so we have to fast forward a decade to find any examples. Suspended MTB's? Add a few more years. If we are talking classic aluminum-framed bikes we'll have to change the date yet again. CF? keep adding years. Pretty soon we get "vintage X" items that were made in the mid/late 00's.

At least bikes have been around for a while. What defines a classic/vintage cell phone? What defines a classic/vintage smart phone? I've got a vintage iPhone...

Give me a break. Anything made past 1980-85 isn't really a vintage anything. It's just nouveau scrap.