I like the Classic Rendezvous definition, though I would include late 70's / early 80's "pioneering era" mountain bikes as well.

I think modern bikes can be "classic", as alluded in the CR definition - style (i.e. randonneur, porteur, roadster, paint scheme, etc) and methods (i.e. lugged frames) can make a modern bike "Classic" in my view. So, I may deviate from some in this regard, as their definition of "Classic" may be specifically attributed to bikes from a certain era in a certain style.

Perhaps the term "retro" is a more appropriate/correct term for modern bikes made as I've described above, but I tend to use "Classic" when my internal editor is switched off.

I define "vintage" as anything older than 20-25 years old, so it is a constantly moving scale.