Classic can be anytime. A relatively modern frame that is recognized as superior, in demand, different from the rest (others will add their own requirements) can be considered a classic. Hence the term "instant classic", although in my opinion that phrase is overused.
In the past the term "Vintage" applied to bikes that were at least 25 years old, but this is general bench mark. As an example, if someone owns a 1991 steel Trek frame and says its a vintage frame, I wouldn't expect anyone to argue that it would have to wait another year. Perhaps the number 25 years has changed, but that used to be the benchmark.
An example of a classic might be some of the higher end Lemond steel frames from the mid 90s. Not yet 25 years old but considered desirable and known to be good performing bikes (wish I had one).