I believe that antique, these days, might be pre-WW1, and perhaps a year or 2 thereafter, 1920(?); but perhaps not to include designs that continued essentially unchanged 10-20 years later. (If a bike requires special parts not interchangeable with much of anything else, then I lean more towards the antique category).
Vintage may include 1980-90’s.
I believe that Classic has been defined by the NBHAA(?), as 1930’s streamlined through early 1960’s cantilever frames; (not sure if the “space cadet” bikes count).
For the 1960’s through 1970’s, (lost decades or beginning of the end?) and 2000’s+, I consider as just old or older bikes.