This is obviously going to be very subjective...but here's my take.

Classic isn't a function of age, though age can be a contributing factor. Bikes that are classic are the bikes that we drool over and likely can't afford when new. They're the best of the best. They began a style or were representative of that style. Classic bikes are the ones that we'd have bought if money wasn't an object and we purchased only based on our emotions. Classic bikes are often the ones that defined a period of cycling...for instance, Armstrong on his Madone is an iconic image. Classic bikes are the ones kids will want when they get middle aged and become collectors.

Classics are timeless, beautiful and cross cycling niches in their appeal. They have something that differentiates them from the other bikes on the market. They are the best in class.